akbasic/TODO.md

# Record

**Outstanding work is in the issue tracker, not in this file:**
<https://source.starfort.tech/andrew/akbasic/issues>

This file was the implementation plan for the Go → C port of `deps/basicinterpret`, written to be
executed by AI agents rather than read for inspiration. **The port is done**, so what it is now is
the record: the design decisions that are settled, the deviations from the reference and why each
was taken, the defects that were found and fixed, and the reasoning behind the measurements.

Each open item that moved leaves a line saying what it was and which issue carries it, because an
entry explaining *why* a defect matters is worth keeping beside the work it constrains — but the
tracking happens there, not here.

Issues are labelled by kind and blast radius, and milestoned by what they can land in: `0.1.x` for
anything that changes no public contract, `0.2.0` for new verbs and observable behaviour changes,
`1.0.0` for the design decisions. Everything filed carries `status::grooming`.

Two gaps this file recorded and deliberately did not file — the missing `HUD` anchors and a
dismissable dialog — are now `libakgl` #79 and #80. §7's rule is right that *changing* a dependency
is that repository's decision; it does not follow that reporting the gap is.

---

## 0. Agent protocol

### 0.1 The Go reference is deprecated. Stop matching it.

**`deps/basicinterpret` is a dead project. It will not be updated, and this interpreter is no
longer required to reproduce its behaviour.** Recorded here first because it silently reverses
the premise several sections of this file were written on, and because an agent that reads them
without this will park work that is no longer blocked.

What it changes:

- **§6 is now an ordinary defect list.** "Port the behaviour first so the port is provably
  faithful" is retired. Fix them because they are wrong, not when fidelity permits.
- **§1.8's message-text contract is now a convention.** Improving a message is allowed; it costs
  a golden file, which is a cost rather than a veto.
- **§5's bar drops** from "defensible against the golden suite" to defensible on its own merits.
- **`tests/language/` is the editable language corpus rather than a protected specification.** Diverging from
  it is allowed and must be deliberate and recorded — see its README.

What it does **not** change:

- The corpus stays and stays green. Forty-one real BASIC programs with known-good output are
  worth having whatever their provenance, and an unexplained change there is still a red flag.
- The Go source stays readable as documentation. It remains the best answer to "what did the
  original actually do here", which is a question worth being able to answer even once the
  answer stops being binding.
- Nothing about the `ak*` house rules, which never came from the reference.

**Read these before touching anything**, in this order:

1. `MAINTENANCE.md` in this repository — project goals, the `libakerror` convention, the
   error-code range map, and the dependency versions.
2. `deps/libakerror/AGENTS.md` — the `ATTEMPT`/`CLEANUP`/`PROCESS`/`HANDLE`/`FINISH` protocol.
3. `deps/libakerror/UPGRADING.md` — 1.0.0's status registry. Required before writing an error
   code; the mechanism it replaced is gone.
4. `deps/libakstdlib`'s issue tracker — the defects and gaps in the library this port calls
   into. §1.9 below says which calls are cleared for use; that section is not optional
   reading, it bans a family of functions the port would otherwise reach for by reflex.
5. `deps/libakgl/AGENTS.md` — the no-`malloc` rule and the commit co-author requirement.
6. `deps/basicinterpret/README.md` — the language reference and the unimplemented list. Still
   worth reading for the verb set and the semantics; no longer binding, per §0.1.

**Rules for working this file:**

- **Do not** mark an item done until its acceptance command passes on a clean out-of-tree
  build. "It compiles" is not acceptance.
- **Do** update the tracker in the same commit as the work: close the issue, or replace it with
  the defect it uncovered. Outstanding items live there; this file holds the record.
- **Do** add the agent program name, model name and version as a commit co-author. That rule
  comes from `libakgl` and applies here.
- **Never** hand-edit generated output. `build/` trees and the generated `akerror.h` are
  off-limits; change the generator.
- **Never** reformat a file you are not otherwise changing.
- When a step is blocked because `libakgl` cannot supply a capability, **do not work around
  it here.** File it against `libakgl` in its issue tracker -- what the BASIC verb requires,
  what the `akgl_*` entry point should look like, and what tests would cover it -- and note the
  block in §7 below.

**Style**, restated so nobody has to go look: C99, 4-space indent, tabs at width 8
(`stroustrup`), function-body braces in column 0 on their own line, control braces on the same
line, **always brace**, spaces inside control-flow parens — `if ( x == y ) {`. Pointer star
binds to the identifier: `char *name`. Prefix is `akbasic_` for functions,
`akbasic_TypeName` for types, `AKBASIC_UPPER_SNAKE` for macros. `static` helpers drop the
prefix. Parameter names must match between header and source.

---

## 1. Design decisions already made

These are settled. Do not relitigate them mid-port; if evidence says one is wrong, say so in
a commit that changes it deliberately, and update this section.

### 1.1 Reflection becomes one aligned dispatch table

The Go runtime resolves verbs with `reflect.MethodByName("Command" + NAME)`
(`basicruntime.go:401`), functions with `"Function" + NAME`, and special parse paths with
`"ParseCommand" + NAME` (`basicparser.go:103`). C has no reflection and we are not adding
any. Replace all three with **one** static table in `src/verbs.c`, sorted by name, searched
with `bsearch(3)`:

```c
/* name        token type              parse handler          exec handler        */
{ "AUTO",      AKBASIC_TOK_CMDIMM,     NULL,                  cmd_auto      },
{ "DATA",      AKBASIC_TOK_COMMAND,    parse_data,            cmd_data      },
{ "DEF",       AKBASIC_TOK_COMMAND,    parse_def,             cmd_def       },
```

A `NULL` parse handler means "parse the rval as a plain expression", which is exactly what
`commandByReflection` returning `(nil, nil)` means today. Adding a verb is adding one row plus
two functions. **Keep the table column-aligned and one row per verb** — it is a table, so it
gets laid out as one.

This also kills the Go scanner's three separate maps (`reservedwords`, `commands`,
`functions`, `basicscanner.go:64-67`): the token type lives in the same row.

### 1.2 Strings are fixed-size and live inline

`libakstdlib` has no string type. `libakgl` has `akgl_String` but it is `PATH_MAX` bytes,
refcounted, and pool-allocated — wrong shape for a value that gets copied on every
assignment, and it would drag a `libakgl` dependency into the core interpreter.

Define in `include/akbasic/types.h`:

```c
#define AKBASIC_MAX_STRING_LENGTH    256    /* matches AKBASIC_MAX_LINE_LENGTH */
```

and give `akbasic_Value` a `char stringval[AKBASIC_MAX_STRING_LENGTH]` **inline**. `clone()`
becomes a struct assignment. No allocator, no refcount, no lifetime question.

**Tradeoff, stated:** every `akbasic_Value` is ~300 bytes, so one environment's
`values[AKBASIC_MAX_VALUES]` pool is ~19KB, and 32 environments is ~610KB of BSS. That is
fine on a PC and is the price of never calling `malloc`. If it ever isn't fine, the knob is
`AKBASIC_MAX_STRING_LENGTH`, not the allocator.

Truncation is an **error**, not a silent clamp: `FAIL_RETURN(e, AKBASIC_ERR_VALUE, ...)`.

### 1.3 Maps become fixed-capacity open-addressed tables

Five Go maps need replacing:

| Go site | Purpose | C replacement |
|---|---|---|
| `BasicScanner.reservedwords/commands/functions` | keyword → token type | the §1.1 static table + `bsearch` |
| `BasicEnvironment.variables` | name → `*BasicVariable` | `akbasic_SymbolTable`, capacity `AKBASIC_MAX_VARIABLES` |
| `BasicEnvironment.functions` | name → `*BasicFunctionDef` | `akbasic_SymbolTable`, capacity `AKBASIC_MAX_FUNCTIONS` |
| `BasicEnvironment.labels` | name → line number | `akbasic_SymbolTable`, capacity `AKBASIC_MAX_LABELS` |

One implementation, `src/symtab.c`, keyed by `aksl_strhash_djb2()` (already in
`libakstdlib`) with linear probing and a fixed slot array. **Use the existing hash; do not
write another one.** Table full is an error, not a resize.

Caveat, recorded upstream: the wrapper sign-extends `char`, so a
high-bit byte hashes wrong — `"\xff\xfe"` returns 5859874 where the `unsigned char` answer is
5868578. BASIC identifiers are 7-bit ASCII (the scanner only accepts `IsLetter`/`IsDigit` plus
a type suffix), so this cannot bite the symbol tables. It **would** bite if anyone later keys
a table on a string literal or a filename. Do not work around it here; it is already filed
upstream.

### 1.4 Environments come from a pool and are released

Go calls `new(BasicEnvironment)` at `basicruntime.go:121` and `basicparser_commands.go:124`
and never frees one. A long-running `GOSUB` or `FOR` in Go leaks; the GC eventually catches
some of it, and nothing in the tests notices.

C gets `HEAP_ENVIRONMENT[AKBASIC_MAX_ENVIRONMENTS]` with
`akbasic_env_acquire()` / `akbasic_env_release()`, in the shape of `akgl_heap_next_*`.
`akbasic_runtime_prev_environment()` **must** release the environment it pops. Pool
exhaustion is `AKBASIC_ERR_ENVIRONMENT`, reported with the current line number.

Watch the one place this is not a clean stack: `userFunction` (`basicruntime.go:348`) stores
a `BasicEnvironment` **by value** inside `BasicFunctionDef` and re-`init()`s it on every call.
In C the funcdef holds an `akbasic_Environment *` acquired at `DEF` time and reset per call —
it is owned by the funcdef, not the pool's free list, until the funcdef dies.

### 1.5 Output goes through a text sink backend

`Write()` and `Println()` (`basicruntime_graphics.go:140,148`) mirror every line to stdout
*and* to an SDL surface. That mirror is the only reason the golden-file suite works. Do not
reproduce it as a hardcoded pair of calls.

Define a record of function pointers, populated by an initializer — the house pattern:

```c
typedef struct akbasic_TextSink
{
    void *self;
    akerr_ErrorContext AKERR_NOIGNORE *(*write)(struct akbasic_TextSink *self, char *text);
    akerr_ErrorContext AKERR_NOIGNORE *(*writeln)(struct akbasic_TextSink *self, char *text);
    akerr_ErrorContext AKERR_NOIGNORE *(*readline)(struct akbasic_TextSink *self, char *dest, size_t len);
    akerr_ErrorContext AKERR_NOIGNORE *(*clear)(struct akbasic_TextSink *self);
} akbasic_TextSink;
```

`akbasic_sink_init_stdio()` is in the library and `akbasic_sink_init_akgl()` is in the
akgl-backed module. The driver is supposed to pick: a default build selects stdio, while an
`AKBASIC_WITH_AKGL` build selects the SDL text path and mirrors its output to stdout. It does
not do that yet; §3 records the missing standalone frontend. Cursor arithmetic, wrapping and
scrolling belong to the akgl sink, not to the interpreter.

### 1.6 The interpreter steps; it does not run

Go's `run()` (`basicruntime.go:682`) is a `for {}` that owns the process until `MODE_QUIT`.
Goal 3 forbids that: a host game must be able to bound execution.

The library exposes:

```c
akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_step(akbasic_Runtime *obj);
akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_run(akbasic_Runtime *obj, int maxsteps);
```

`_step()` does exactly what one iteration of Go's `for {}` body does and returns.
`_run(obj, maxsteps)` loops `_step()` until the mode is `AKBASIC_MODE_QUIT` or `maxsteps`
steps have elapsed; `maxsteps <= 0` means unbounded, which is what the standalone driver
passes. **This is a deliberate restructure, and it must not change a single byte of golden
output.**

`QUIT` sets `AKBASIC_MODE_QUIT` and returns. **Nothing in the library calls `exit()`,
`abort()`, or `FINISH_NORETURN`.** `FINISH_NORETURN` appears exactly once in the tree, in
`src/main.c`.

### 1.7 Error codes are absolute, reserved from the 1.0.0 registry

`deps/libakerror` is at **1.0.0**. Read `deps/libakerror/UPGRADING.md` before writing an error
code; `MAINTENANCE.md`'s error-code section is the condensed version. Three things this changes
from what you may have seen in an older draft or in `libakgl`:

- `AKERR_MAX_ERR_VALUE` **no longer exists**. The name registry is sparse and takes any `int`.
  There is nothing for a consumer to size, and no compile definition to set. Anywhere you find
  one, delete it.
- `__AKERR_ERROR_NAMES` **no longer exists**. The table is private.
- Codes are **absolute integer constants at 256 or above**, never `AKERR_LAST_ERRNO_VALUE + N`.
  The offset scheme is what produced the live `libakgl` collision documented in
  `MAINTENANCE.md`.

`akbasic` owns **512–767** per the range map in `MAINTENANCE.md`. Declare it as an `enum`, so the
values stay compile-time integer constants (`HANDLE` expands to `case` labels, which require
that) and adding a code does not mean renumbering an offset:

```c
#define AKBASIC_OWNER "akbasic"

enum {
    AKBASIC_ERR_BASE        = 512,   /** Start of akbasic's reserved status range */
    AKBASIC_ERR_SYNTAX      = AKBASIC_ERR_BASE,
                                     /** Parse-time grammar violation */
    AKBASIC_ERR_TYPE,                /** Incompatible types in an operation */
    AKBASIC_ERR_UNDEFINED,           /** Reference to an undefined verb, function or label */
    AKBASIC_ERR_BOUNDS,              /** Array subscript or pool index out of range */
    AKBASIC_ERR_ENVIRONMENT,         /** Environment pool exhausted or orphaned environment */
    AKBASIC_ERR_VALUE,               /** A value was malformed, truncated or unconvertible */
    AKBASIC_ERR_STATE,               /** A verb ran outside the block structure it requires */
    AKBASIC_ERR_LIMIT       = AKBASIC_ERR_BASE + 256
};
```

`akbasic_init()` reserves the **whole** 256 in one call and then names each code. Both
registry calls return `akerr_ErrorContext *` and are `AKERR_NOIGNORE`, so a collision is an
ordinary error — `PASS` it and let it propagate out of init:

```c
akerr_ErrorContext AKERR_NOIGNORE *akbasic_init(void)
{
    PREPARE_ERROR(errctx);

    PASS(errctx, akerr_reserve_status_range(AKBASIC_ERR_BASE,
					    AKBASIC_ERR_LIMIT - AKBASIC_ERR_BASE,
					    AKBASIC_OWNER));
    PASS(errctx, akerr_register_status_name(AKBASIC_OWNER, AKBASIC_ERR_SYNTAX,
					    "Syntax Error"));
    /* ... one per code ... */
    SUCCEED_RETURN(errctx);
}
```

Reserve the whole range in **one** call — a subset or superset of your own range raises
`AKERR_STATUS_RANGE_OVERLAP`, not a no-op. Use `akerr_register_status_name()`, never the
two-argument `akerr_name_for_status(status, name)` set path: the owned form is the one that
catches a component writing into a range that is not its own, and the two-argument form is
what let `libakgl` silently clobber `libakerror`'s names.

There is no startup ceiling to assert any more — the BSS-overflow hazard the old draft
defended against was deleted along with `AKERR_MAX_ERR_VALUE`. What replaces it is the
reservation itself: if `akbasic_init()` returns an error, something else owns part of 512–767
and the process must not continue as though it does not.

Do not call `akerr_init()` first. Every registry entry point calls it, and since 1.0.0 it no
longer clears reservations made before it ran.

### 1.8 Error message text is a convention, no longer a contract

`tests/language/array_outofbounds.txt` is, verbatim:

```
? 20 : RUNTIME ERROR Variable index access out of bounds at dimension 0: 4 (max 2)\n\n
```

The trailing double newline is real: `basicError` builds a string ending in `\n` and hands it
to `Println`, which adds another.

**This used to be a hard contract and is now a default.** The Go implementation is deprecated
and will not be updated, so the two projects are no longer required to match — see §0.1. What
survives is the practical half: these strings and this newline behaviour are what every
expectation in `tests/language/` was written against, so changing one means changing the paired
files, and that is worth doing on purpose rather than by accident. A message that reads
awkwardly *may* now be improved; do it deliberately, move the expectations in the same commit,
and add a line to §5.

Numeric formatting still matches the reference: integers via `%" PRId64 "`, floats via `%f`
(Go's `%f` and C's `%f` both give six decimals — `tests/language/arithmetic/float.txt`
confirms). No reason to change it, which is different from not being allowed to.

### 1.9 Which `libakstdlib` calls are cleared for use — **the bans are lifted**

`deps/libakstdlib` is at **0.2.0**, and that release *fixed all six* of the confirmed defects
this section was built around, plus seventeen contract gaps. The table of bans that used to
live here is gone because it is no longer true, and leaving it would send an agent around a
workaround for a function that now works.

What changed, in the four that mattered to this port:

| Was banned | Now |
|---|---|
| `aksl_atoi` / `_atol` / `_atoll` / `_atof` | Every one takes a `dest` pointer and raises `AKERR_VALUE` on no digits or trailing junk, `ERANGE` on overflow — exactly the contract this section demanded |
| `aksl_list_append` / `_iterate` | Append no longer truncates to two nodes; iterate no longer skips the first half |
| `aksl_tree_iterate` | `AKERR_ITERATOR_BREAK` stops the traversal |
| `aksl_realpath` | Rewritten; no longer reads uninitialised memory on its error path |

Two signature changes reach this repository. `aksl_fread` and `aksl_fwrite` now take a
required `size_t *nmemb_out` and report a short transfer as `AKERR_IO` instead of a silent
success — so a `DSAVE` onto a full disk is now an error a program sees, where before it
reported nothing. `src/runtime_commands.c` passes the count and discards it, which is correct:
the library does the noticing now.

**`src/convert.c` is gone.** It existed only because `aksl_ato*` could not report a conversion
failure, and its own note said what to do when that changed: *"When it grows it, delete
`src/convert.c` and switch the call sites over."* That is done. Six call sites now go straight
to the library — `newLiteralInt` and `newLiteralFloat` in `src/grammar.c`, the line number in
`src/scanner.c`, `FunctionVAL` in `src/runtime_functions.c`, `INPUT` in
`src/runtime_commands.c`, and `GSHAPE`'s handle in `src/runtime_graphics.c`.

Three things that fell out of it, recorded because each was checked rather than assumed:

- **`aksl_strtoll(str, NULL, base, &dest)` is the exact replacement**, not `aksl_atoll`, wherever
  a base is involved. The `ato*` forms are base 10; `src/grammar.c` picks base 8 or 16 from the
  lexeme's prefix. A `NULL` endptr is what makes trailing junk an error rather than a stopping
  point, and that is the whole contract this port needs.
- **The raised status changed** from `AKBASIC_ERR_VALUE` to `AKERR_VALUE`, and the message text
  with it — `VAL("garbage")` now says `no digits in "garbage"`. §1.8 makes message text part of
  the acceptance contract, so this was checked against the corpus first: **no golden file
  contained a conversion message**, which is also why §1.9 used to ask for one. Two now exist,
  `tests/language/numeric/val_refuses_garbage` and `octal_literal`, so the next change to that
  text has to move a golden file.
- **`tests/convert.c` became `tests/numeric_contract.c`.** The assertions did not become
  worthless when the wrapper went away — they became assertions about a contract this port
  depends on and no longer owns, which is worth pinning exactly because a regression in it would
  make `VAL("garbage")` return 0 silently. Same reasoning `libakstdlib`'s own
  `tests/test_status_registry.c` gives for pinning that it reserves no status range.

One caveat survives the upgrade unchanged: `aksl_strhash_djb2` still sign-extends `char`, so a
high-bit byte hashes differently from the `unsigned char` answer. BASIC identifiers are 7-bit
ASCII so the symbol tables cannot reach it — see §1.3, which is still accurate.

---

## 2. What exists — **the core port is complete and green**

Phases 0 through 6 of the original plan are done. The interpreter builds clean under
`-Wall -Wextra`, passes the reference's entire corpus, and passes under ASan and UBSan.

It *did* reproduce the reference byte for byte, and that claim is retired rather than broken:
§0.1 released it, and one case has since diverged deliberately (§6 item 16, listed in
`tests/language/README.md`). Everything else still matches, which is worth knowing but is no
longer a gate.

```sh
cmake -S . -B build && cmake --build build --parallel
ctest --test-dir build --output-on-failure          # 59/59
cmake -S . -B build-asan -DAKBASIC_SANITIZE=ON      # 59/59
cmake -S . -B build-cov  -DAKBASIC_COVERAGE=ON      # 92.3% line, 96.9% function
```

| Module | Source | Reference |
|---|---|---|
| Symbol table | `src/symtab.c` | the five Go maps |
| Value | `src/value.c` | `basicvalue.go` |
| Variable | `src/variable.c` | `basicvariable.go` |
| Tokens and AST leaves | `src/grammar.c` | `basicgrammar.go` + `basicparser.go` |
| Dispatch table | `src/verbs.c` | the three reflection lookups |
| Scanner | `src/scanner.c` | `basicscanner.go` |
| Parser | `src/parser.c`, `src/parser_commands.c` | `basicparser*.go` |
| Environment | `src/environment.c` | `basicenvironment.go` |
| Runtime | `src/runtime.c` | `basicruntime.go` minus SDL |
| Verbs and functions | `src/runtime_commands.c`, `src/runtime_functions.c` | `basicruntime_{commands,functions}.go` |
| Text sink | `src/sink_stdio.c` | `basicruntime_graphics.go`'s stdout mirror |
| Stdio driver | `src/main.c` | `main.go` minus its SDL frontend |
| Embedding examples | `examples/embed.c`, `examples/hostvars.c` | *(new)* |

`akbasic_runtime_load(rt, source)` was added while writing the README's embedding
section: a host usually holds its script as a string and wants the sink reserved for output,
and the only path that existed — `AKBASIC_MODE_RUNSTREAM` reading through the sink's
`readline` — forces a game to point its output device at its source text. `examples/embed.c`
is the code the README quotes, built by every build and registered as a CTest case so a
signature change breaks the build rather than rotting the document.

**The acceptance suite is the editable language corpus, checked in at `tests/language/`.** All
65 `.bas` files are registered as individual CTest cases and compared against their `.txt`
— including the trailing double newline on an error line (§1.8).

It was driven *in place* out of `deps/basicinterpret` until 2026-07-31, on the reasoning that
copying a submodule's corpus guarantees drift. That reasoning was sound and was overruled
deliberately: the Go dependency is being deprecated, and a build that cannot run its own
acceptance suite without cloning the implementation it replaced is not finished. The copy is
originally byte-identical to `basicinterpreter@d76162c`, and `tests/language/README.md` records the
provenance and the rule that programs and expectations are edited together deliberately.

**Nothing in the build or the suite needs `deps/basicinterpret` any more**, and that is checked
rather than assumed — both configurations were configured, built and run from scratch with the
submodule moved out of the tree. The Commodore font moved too, to `assets/fonts/`, which was the
other thing tying the build to it; `assets/fonts/PROVENANCE.md` carries the licence question that
came with it.

Eighteen unit tests cover the modules the corpus cannot reach on its own.
`tests/known_reference_defects.c` is registered in `AKBASIC_KNOWN_FAILING_TESTS` and asserts
the **correct** contract for six of the defects in §6; when one is fixed CTest reports
"unexpectedly passed", which is the cue to split that assertion out and strike the item.

Two budget numbers moved during implementation and are worth knowing:
`AKBASIC_MAX_ARRAY_ELEMENTS` (1024) caps one array and `AKBASIC_MAX_ARRAY_VALUES` (4096) caps
all of them together, drawn from an `akbasic_ValuePool` the runtime owns. The reference had no
such ceiling because it called `make()`.

---


## 3. The akgl-backed sink, devices and standalone frontend — **done**

`-DAKBASIC_WITH_AKGL=ON` builds and its suite passes, and the build option now changes what
the executable *does*: an AKGL build of `basic` opens a window, draws BASIC output into it,
pumps SDL events, lets you type at it, and still puts every byte on stdout. Four adaptors in
the `akbasic_akgl` target, which is the only thing here that links SDL, are complete and
tested:

| File | Backs |
|---|---|
| `src/sink_akgl.c` | the §1.5 text sink, over `akgl_text_measure` and `akgl_text_rendertextat` |
| `src/graphics_akgl.c` | `akbasic_GraphicsBackend`, over `akgl_draw_*`, and the SSHAPE surface pool |
| `src/audio_akgl.c` | `akbasic_AudioBackend`, over `akgl_audio_*` |
| `src/input_akgl.c` | `akbasic_InputBackend`, over `akgl_controller_poll_key` |

The character grid comes from `akgl_text_measure(font, "A", &w, &h)` — the direct equivalent of
the `font.SizeUTF8("A")` at `basicruntime.go:96`, and the call this was blocked on until
42b60f7. Wrapping is done on the character grid rather than by handing SDL_ttf a `wraplength`,
because the cursor has to land somewhere definite: a program that `PRINT`s a long string and
then `PRINT`s again expects the second to start on the row after the first ended, and only the
code that placed the characters knows which row that is.

**The interpreter owns no window, renderer or event loop.** Every initializer takes something
the host already made. Each calls `akgl_error_init()` first: `akgl_game_init()` would have,
but we drive subsystems directly and never call it, and a code raised before that registration
carries no name into its stack trace. It is idempotent.

*Acceptance:* `tests/akgl_backends.c`, a 128x128 software renderer under the dummy video
driver read back with `SDL_RenderReadPixels` — the pattern `deps/libakgl/tests/draw.c`
established, which needs no display and no offscreen harness. Registered as the `akgl_backends`
CTest case, and only when `AKBASIC_WITH_AKGL` is on.

### The standalone frontend

The standalone executable is itself a host, and that half of the Go frontend is now ported —
`src/frontend_akgl.c`, in its own `akbasic_frontend` target. **The separation is in the build
graph and not only in a comment**: `akbasic` is the interpreter, `akbasic_akgl` is the four
adaptors that draw through somebody else's renderer, and `akbasic_frontend` is the one thing in
the repository that creates a window. A game embedding the interpreter links the first two and
not the third.

| Piece | Where |
|---|---|
| Window, renderer, font, the four adaptors | `akbasic_frontend_akgl_init()` |
| Sink and devices onto a runtime | `akbasic_frontend_akgl_attach()` |
| One frame: pump events, draw the text layer, present | `akbasic_frontend_akgl_pump()` |
| The bounded frame loop | `akbasic_frontend_akgl_drive()` |
| stdout mirror | `src/sink_tee.c`, in the *core* library |

Four things are worth knowing about how it came out.

1. **The stdout mirror is a sink, not a second write.** §1.5 made the sink boundary precisely
   so the interpreter would never carry the reference's hardcoded pair of calls, and §3 item 5
   said it again. `akbasic_sink_init_tee()` composes two sinks into one and takes `readline`
   from whichever of the two was named as the reader — which is the whole difference between
   the two modes: a program read from a file comes in through the stdio half, and typed lines
   come in through the akgl half's line editor. It lives in the core library and is tested
   there (`tests/sink_tee.c`), because composing two function-pointer records needs no SDL.

2. **The line editor borrows the host's loop a frame at a time.** `readline` has to wait for a
   typed line, and §1.6 forbids the library blocking — and a sink that sat on the keyboard with
   no way to pump events would deadlock the process on its first `INPUT`. So the host installs
   an `akbasic_AkglPump` with `akbasic_sink_akgl_set_pump()`, and the editor calls it between
   keystrokes. The loop is still the host's. `akbasic_frontend_akgl_pump()` has that exact
   signature and is installed as-is. Without a pump the sink reports EOF, which is what it did
   before and what a host that wants no editor still gets.

3. **The frame loop is bounded, and that is what makes the close button work.** 256 steps per
   frame (`AKBASIC_FRONTEND_STEPS_PER_FRAME`), then pump and present. `10 GOTO 10` under an
   unbounded `run()` would own the process forever; `tests/akgl_frontend.c` runs exactly that
   and asserts the window close ends it.

4. **The frame is not cleared.** The graphics verbs draw straight to the renderer rather than
   into a display list, so clearing would wipe every `DRAW` at the end of the frame it was
   issued in. The text layer is drawn over whatever is there, which is also how a C128 stacks
   its text plane on its bitmap plane.

*Acceptance:* two things, and the second one is the stronger.

- `tests/akgl_frontend.c`, registered as the `akgl_frontend` CTest case: a program run through
  the frontend under the dummy driver, asserting the mirrored stdout **byte for byte** and
  reading the renderer back to prove the same characters were drawn in the Commodore font;
  synthetic key events pushed into SDL's own queue and read back through the frontend's input
  backend, so every link the host owns is in the path; the line editor's fold to upper case,
  its backspace and its escape; a whole REPL session typed at the window and ended with a typed
  `QUIT`; and the window-close path.
- **The entire golden corpus is driven through the AKGL binary.** A `-DAKBASIC_WITH_AKGL=ON`
  build registers the same 41 upstream cases against the same executable, which now opens a
  window to run them, and all 41 still match byte for byte. That is a much broader statement
  than any hand-written frontend test: the SDL path changes no observable output anywhere in
  the corpus.

Three `no_device.bas` local cases are **deliberately not registered** in an AKGL build. Their
premise is a driver that was given no devices, which is true of the stdio driver and
deliberately false of this one. The refusal path they cover is asserted directly against the
backend records in `tests/devices.c`, which both configurations build.

**Manual check against a real display, 2026-07-31.** The dummy driver cannot prove presentation,
so `build-akgl/basic` was run against X display `:0` on a program that prints a line, draws a
`BOX` and then loops. `xwininfo` found a real 800x600 window titled `BASIC`; the window was
captured with `import` and measured: the first text row carries glyph pixels (mean 65/255), the
box's left edge at x=100 is a solid white line (mean 255), the box interior is black — it
outlines rather than fills — and an empty corner is black. stdout carried `HELLO WORLD` at the
same time. `QUIT` exits cleanly with status 0.

**The typing half is no longer manual, and it is no longer a gap.** It used to read "somebody
should type at it once", because no key-synthesis tool was installed. `xdotool` now is, and
`tests/akgl_typing.sh` is registered as the `akgl_typing` CTest case: it starts the driver under
a pty, waits for the window with `xdotool search --sync`, gives it keyboard focus, types a
program containing a **string literal** and a **lower-case** one, and polls the mirrored stdout
for what they print.

That case earns its keep twice over. It is the only test that covers the path *upstream* of SDL
— X11 → SDL composition → libakgl's ring → the editor — and everything else synthesises SDL
events, which is precisely how the missing `SDL_StartTextInput()` shipped with a green suite.
Confirmed by reverting both halves of that fix and watching it fail with the same symptom that
was reported: `READY`, and nothing after it.

It needs a real X server, a window manager and `xdotool`, and it steals keyboard focus for about
fifteen seconds. Absent any of those it reports CTest `Skipped` rather than failing, which is
what it does in CI. `AKBASIC_SKIP_INTERACTIVE=1` skips it deliberately.

### The five things that had to be worked around — **all gone**

Every one was filed against `libakgl` and every one is resolved in its 0.3.0. What is left here
is the note that they existed, because the pattern is the point: file it upstream, comment the
workaround at its site with the words "filed upstream", delete it when it lands.

| Was | Resolved by |
|---|---|
| An embedded `libakgl` required its vendored SDL, SDL_image, SDL_mixer, SDL_ttf and jansson to be *installed*, so our `CMakeLists.txt` declared all five by hand first | It adds them whether or not it is top-level |
| `akgl/controller.h` did not compile on its own — it used `akgl_Actor *` and included nothing that declared it | It includes `<akgl/actor.h>` |
| There was no way to bind a 2D backend to a renderer you already had, so the six vtable pointers were assigned by hand in two files | `akgl_render_bind2d()` |
| `akgl_text_rendertextat()` dereferenced an unset `draw_texture`, so the first `PRINT` through an unbound backend segfaulted | It checks, and reports `AKERR_NULLPOINTER` like the draw entry points |
| `SOUND`'s frequency sweep had no equivalent and was refused | `akgl_audio_sweep()` |

**0.3.0 also arrived with a regression that this repository is the one that found**, because it
is the only consumer that embeds `libakgl` alongside other projects: the commit that made the
vendored dependencies unconditional also made its `add_test()` shadow unconditional, and CMake
chains command overrides exactly one level deep. Two projects in one tree cannot both shadow
`add_test` — the second rebinds `_add_test` to the first and the builtin becomes unreachable to
everybody, so our own registrations recursed to CMake's depth limit. Fixed upstream by putting
the top-level guard back; filed as `libakgl` defect 27, with the two-line CMake program that
demonstrates the one-level limit.

### Still missing from the sink

Nothing that blocks a verb, and as of `libakgl` 0.3.0 nothing that blocks a *character* either.
The ring now carries the composed UTF-8 text SDL worked out from each keystroke, so the editor
takes that in preference to the keycode and shifted characters, keyboard layouts, compose keys
and dead keys all work. **A double quote can be typed, so a BASIC string literal can be typed**,
which was the sharp end of the old limitation — it used to mean a program with a string in it
had to be passed on the command line. Letters are no longer folded to upper case, because there
is no longer any reason to.

**The host has to turn text input on**, and forgetting to is how this broke once. SDL3 has it
off by default and per-window, so it is `akbasic_frontend_akgl_init()` that calls
`SDL_StartTextInput()` — `akgl/controller.h` says as much. Without it SDL emits no
`SDL_EVENT_TEXT_INPUT` at all, every keystroke reaches the ring with an empty `text`, and an
editor that reads that as "not a character" is silently dead: nothing echoes in the window, and
nothing reaches stdout either, because `RUN` can never be typed.

The editor therefore **falls back to the keycode** when a keystroke carries no composed text,
folded to upper case. A worse keyboard is a great deal better than no keyboard, and it means a
host that embeds these adaptors and forgets `SDL_StartTextInput()` gets a usable editor rather
than a dead one. Both halves are asserted in `tests/akgl_frontend.c`, and both were checked by
reverting each in turn and watching the test fail.

Worth knowing about the test suite that missed this: every other keyboard test pushes
`SDL_EVENT_TEXT_INPUT` into SDL's queue by hand, which is what a real keyboard produces — *once
text input has been started*. Synthesising the end of a chain cannot test the beginning of it.
The new test asserts `SDL_TextInputActive()` directly for that reason, and the whole frontend
suite has been run against a real X11 window as well as the dummy driver.

One limit remains, and it is a choice rather than a gap:

- **No cursor movement within a line.** Backspace and escape only. The arrow keys stay in the
  ring for a script's own `GET` loop, which is the more valuable use of them.

A non-ASCII character is accepted by the keyboard and then dropped rather than stored, which is
§1.2 rather than the editor: the grid is a byte per cell and a value's string is a fixed 256
bytes, so a multi-byte character has nowhere to go. Dropping it is honest where storing half of
it is not.

`WINDOW` and `KEY` in group E want a text-window rectangle and a function-key table on top of
this, which is ordinary work here rather than anything blocked.

---

## 4. Remaining work: language completion (goal 2)

The work queue is the "What Isn't Implemented" list in `deps/basicinterpret/README.md`.
Each verb is: table row (§1.1) → parse handler if it needs one → exec handler → unit test →
a new `.bas`/`.txt` pair. **Both** kinds of test; they answer different questions.

Order by what unblocks the most, and by what does not need `libakgl` to grow first:

| Group | Verbs | Blocked on |
|---|---|---|
| ~~A. Structure~~ | ~~`DO`, `LOOP`, `WHILE`, `UNTIL`, `ON`, `BEGIN`, `BEND`, `END`~~ | **done** — `src/runtime_structure.c`, `tests/structure_verbs.c` |
| ~~B. Housekeeping~~ | ~~`NEW`, `CLR`, `CONT`, `SWAP`, `TRON`, `TROFF`, `HELP`~~ | **done** — `src/runtime_housekeeping.c`, `tests/housekeeping_verbs.c` |
| ~~B′. Housekeeping, deferred~~ | ~~`RESTORE`, `RENUMBER`~~ | **done** — `src/data.c` and `src/renumber.c`, `tests/read_data.c` and `tests/renumber.c` |
| ~~C. Errors~~ | ~~`TRAP`, `RESUME`, `ER`, `ERR`~~ | **done** — `src/runtime_trap.c`, `tests/trap_verbs.c`. `ER`/`EL` are `ER#`/`EL#`; see §5 |
| ~~D. Strings/format~~ | ~~`USING`, `PUDEF`, `WIDTH`, `CHAR`~~ | **done** — `src/format.c`, `src/runtime_format.c`, `tests/format_verbs.c` |
| ~~E. Console~~ | ~~`WINDOW`, `KEY`, `SLEEP`, `WAIT`, `TI`~~ | **done** — `src/runtime_console.c`, `tests/console_verbs.c` |
| ~~F. Disk~~ | ~~all 21~~ | **done** — `src/runtime_disk.c`, `tests/disk_verbs.c`. Five refuse for want of a drive and `DIRECTORY` for want of an upstream wrapper; see §5 |
| ~~G. Graphics~~ | ~~`GRAPHIC`, `DRAW`, `BOX`, `CIRCLE`, `PAINT`, `COLOR`, `SCALE`, `SSHAPE`, `GSHAPE`, `LOCATE`~~ | **done** — `src/runtime_graphics.c`, `tests/graphics_verbs.c` |
| ~~H. Sprites~~ | ~~`SPRITE`, `MOVSPR`, `SPRCOLOR`, `SPRSAV`, `COLLISION`~~ | **done** — `src/runtime_sprite.c`, `src/sprite_akgl.c`, `tests/sprite_verbs.c`. `SPRDEF` is out of scope; see below |
| ~~I. Audio~~ | ~~`PLAY`, `SOUND`, `ENVELOPE`, `VOL`, `TEMPO`~~ | **done** — `src/runtime_audio.c`, `src/play.c`, `tests/audio_verbs.c` |
| I′. Audio, still gapped | `FILTER` | `libakgl` has no filter stage and SDL3 supplies no primitive; refused with `AKBASIC_ERR_DEVICE`. `SOUND`'s sweep arguments used to be here and landed with `akgl_audio_sweep` in libakgl 0.3.0 |
| ~~E′. Console input~~ | ~~`GET`, `GETKEY`, `SCNCLR`~~ | **done** — `src/runtime_input.c`, `tests/input_verbs.c` |
| ~~J. Machine~~ | ~~`SYS`, `FETCH`, `STASH`~~ | **done** — `src/runtime_machine.c`, `tests/machine_verbs.c`. `SYS` is refused by name; see §5 |

**`RESTORE` and `RENUMBER` are deferred, and neither is a small job.**

- ~~**`RESTORE` needs a DATA pointer, and there isn't one.**~~ **Done.** Every `DATA` statement
  is pre-scanned into one flat list before the program runs -- `src/data.c`, called from
  `akbasic_runtime_set_mode()` beside the label prescan -- and `READ` walks a cursor along it.
  `RESTORE` resets the cursor; `RESTORE (line)` moves it to the first item on or after that
  line.

  It fixed two defects on the way, both of which were consequences of `READ` not reading.
  **A `DATA` line above its `READ` is now found**, where skipping forward from the `READ` used
  to run off the end of the program in silence. And **the lines between a `READ` and its `DATA`
  now execute**, where the skip used to swallow them -- a C128 runs them, because `READ` takes
  the next item and execution carries on. The one corpus case has its `DATA` immediately after
  its `READ`, so neither was observable there. `tests/read_data.c`.

  `DATA` at run time is now a no-op: it is a declaration, and reaching the statement means
  walking past it.

- ~~**`RENUMBER` has to rewrite references or it is worse than useless.**~~ **Done.**
  `src/renumber.c` builds the whole old-to-new map first, then rewrites every line -- including
  lines *before* the renumbered region, which can branch into it -- and only then moves them.

  The rewrite is textual and understands exactly one thing about the rest of the language:
  where a string literal starts and stops, so `PRINT "GOTO 10"` survives. `GOTO`, `GOSUB`,
  `RUN`, `RESTORE` and `TRAP` targets are rewritten, and so is the comma-separated list after
  `ON X GOTO` -- for free, since the targets follow the `GOTO`. `COLLISION` is special-cased
  because its handler is its *second* argument.

  Two deliberate non-rewrites. **A target naming a line that does not exist is left alone**:
  `GOTO 9999` in a program with no line 9999 is already broken, and inventing a destination
  would hide that. And **`THEN`/`ELSE` do not take bare line numbers in this dialect** -- the
  corpus writes `THEN GOTO 100` -- so a number after `THEN` is an expression, not a target.

  A renumbering that would land a moved line on a kept one is refused before anything is
  written, because losing a line to a renumbering is not recoverable.

  **A program written with labels needs none of this.** `GOTO DONE` is unaffected by any
  renumbering, which is the strongest argument for `LABEL` there is.

`LOCATE` used to appear in both E and G. It belongs to G alone: in BASIC 7.0 `LOCATE` moves the
*graphics* pixel cursor that `DRAW` starts from, and the text cursor verb is `CHAR`, which is
already in group D.

**Out of scope, and staying that way** — the reference marks these as incompatible with a
modern PC and that reasoning stands: `BANK` (no bank switching), `FAST` (irrelevant CPU speed
control), `MONITOR` (no machine-language monitor). **`SPRDEF` joins them** on the same
reasoning: it is not a programmable verb but an interactive full-screen sprite editor, driven
by single keystrokes and its own cursor. A program cannot call it usefully and this interpreter
does not own the screen it would take over. The three `SPRSAV` source forms are what replace it.

**The `libakgl` JSON layer turned out not to be in the way at all.** Group H was parked on
"check the sprite/actor API before filing", and the concern was that libakgl's sprites are
described by JSON documents that would be cumbersome to reach through BASIC.
`akgl_sprite_load_json()` is a thin wrapper over `akgl_spritesheet_initialize` +
`akgl_sprite_initialize` + writes to public struct fields, and every field it fills from a
document — frame list, animation speed, loop flags, state-to-sprite map — is something a
Commodore sprite does not have. So `src/sprite_akgl.c` builds the same four objects directly.
The one field that *did* survive into the verb syntax is the spritesheet's filename:
`SPRSAV "ship.png", 1` goes through `akgl_path_relative()` and `akgl_spritesheet_initialize()`,
which is the same pair of calls the JSON loader makes.

Also on the queue, from the reference's own defect list:

- ~~**Multiple statements per line**~~ **Done.** `akbasic_parser_parse()` consumes a run of
  `COLON` tokens before each statement and hands the caller a NULL leaf when nothing followed
  them, which is how an empty statement — a trailing separator, or `::` — stays legal rather
  than becoming an error. `tests/parser_commands.c` counts statements per line;
  `tests/language/statements/multiple_per_line.bas` asserts what actually runs.

  **One thing about it needed a decision rather than a transcription**, and it is recorded as
  deviation 31 in §5: BASIC 7.0 scopes everything after `THEN` to the condition, so
  `IF C THEN A : B` must run neither `A` nor `B` when `C` is false. The parser takes exactly one
  statement per arm, so the rest of the line reaches the statement loop as ordinary statements
  and the branch has to tell the loop whether to run them.

  **A known limitation, not a defect:** a whole `FOR`/`NEXT` on one line
  (`FOR I# = 1 TO 3 : PRINT I# : NEXT I#`) does not loop. The block structure is the
  reference's `waitingForCommand` model (§1.6), which skips forward *by source line* to the verb
  it is waiting for, so a `NEXT` on the same line as its `FOR` is never reached. Fixing it means
  restructuring control flow to work on statements rather than lines, which is a deliberate
  piece of work and wants its own commit. Group A's `DO`/`LOOP` will meet the same wall.
- ~~**Array references in parameter lists**~~ **Done**, and it turned out to be §6 item 13 a
  third time rather than a separate defect. An identifier's subscript list moved to `.expr`, and
  `akbasic_ASTLeaf` grew a `next` field so an argument list chains through a link of its own.
  That last part is the real fix: the reference chains arguments through each argument's own
  `.right`, and *every* leaf type that can be an argument already uses `.right` for something,
  so moving one operand out of the way only shifted the collision along. Covered by
  `tests/language/arrays_in_parameter_lists.bas` and `tests/runtime_evaluate.c`.

### What a demoscene program wanted and could not have

`examples/megademo/megademo.bas` was written to push the interpreter to its edges, and these are
the edges it hit. **None of them blocked the demo** — every one has a workaround and the program
carries all six — but each workaround is a routine every future game will carry too, and that is
the argument for the verb.

Ordered by how much BASIC each one would delete: `RND` and `ASC` (#16), the unwritable palette
(#17), `GSHAPE`'s missing blend modes (#18), no way to ask whether `PLAY` has drained (#19),
`PLAY` being monophonic (#20), `DATA` being too small a pipe for assets (#21), and the `TI#` spin
that starves the audio queue (#22).

**The last of those is the one worth reading before writing a game loop.** Polling `TI#` does not
merely waste CPU: the host calls `settime()` once per `runtime_run()` batch, so a batch of spin
statements drops the note release rate below what a sixteenth-note tune needs, the queue never
reclaims slots until it drains completely, and the backlog compounds until `PLAY queue is full`.
Reproduced both ways in isolation. Sleep a frame instead.

---

## 5. Deliberate deviations from the reference

Keep this list current. Most of these change structure without changing observable output;
the ones that *do* change output say so and carry the golden file they moved.

The bar has dropped since this list was started. It used to be "defensible against the golden
suite", because matching the Go implementation byte for byte was goal 1's headline claim. That
implementation is now deprecated (§0.1), so the bar is the ordinary one: defensible on its own
merits, recorded here, and tested.

1. Reflection → static dispatch table (§1.1).
2. Go maps → fixed open-addressed tables (§1.3).
3. Unbounded `new(BasicEnvironment)` → pool with release (§1.4).
4. Hardcoded stdout+SDL mirror → text sink backend (§1.5).
5. `run()` owns the process → `step()` / bounded `run()` (§1.6).
6. `panic()` → `FAIL_RETURN`; no library call terminates the process (§1.6).
7. `debug.PrintStack()` on parse error → the `akerr` stack trace only (§3.1 of the original plan).
8. `BasicEnvironment.eval_clone_identifiers` deleted as dead state (§4.2 of the original plan).
9. `BasicValue.name` and `BasicEnvironment.update()` deleted as dead: nothing ever writes the
   field a non-empty string, and `update()` — its only reader — has no callers.
10. The `DEF`-statement bootstrap is gone. The reference declares every builtin by *running a
    BASIC program of `DEF` lines through the interpreter* at startup and then nulling out the
    expressions (`basicruntime_functions.go:14`); here a builtin's name, arity and handler are
    a row in the dispatch table, and `MOD`, `SPC` and `STR` are ordinary native handlers. This
    removes the need to run the interpreter before the interpreter is ready.
11. Undefined behaviour the reference reaches by a defined route is refused rather than
    inherited: a shift count outside 0..63, a negative string multiplier, and integer division
    by zero all raise. Go defines all three (or panics); C does not. No golden case exercises
    any of them, so observable behaviour is unchanged.
12. `CommandEXIT` clears the pending `NEXT` wait before popping. The reference does not, which
    leaves the parent waiting for a `NEXT` that never arrives (§6 item 8) — in C that is a hang
    rather than a misbehaviour, and no golden case depends on it.

### Deviations in the verbs the reference never implemented

Items 1–12 above are deviations *from ported code*. These are deviations from **Commodore
BASIC 7.0**, in verbs the reference lists as unimplemented and which therefore had no Go
behaviour to port. They matter to somebody typing in a listing out of a C128 manual.

13. **Hardware verbs reach a device through a backend record, and refuse when there is none.**
    `akbasic_GraphicsBackend`, `_AudioBackend` and `_InputBackend` are records of function
    pointers on the runtime; all three may be `NULL`, which is what the current stdio-only
    standalone driver gives them. That is correct for a default build and unfinished for an
    `AKBASIC_WITH_AKGL` build; see §3. A verb that needs a device it was not given raises
    `AKBASIC_ERR_DEVICE` naming itself. `COLOR`, `LOCATE`, `SCALE`, `ENVELOPE`, `TEMPO` and
    `VOL` deliberately do *not* require one — they change interpreter state, so a program can
    configure itself before the host lends it a renderer.

14. **`CIRCLE` is a polygon, always.** BASIC 7.0's `CIRCLE` takes two radii, a start and end
    angle, a rotation and a degree increment, which makes it an inc-degree polygon by
    definition. `akgl_draw_circle` exists and is deliberately **not** used: it draws one
    radius, full sweep, unrotated, so it could serve only the case where every optional
    argument is defaulted — and a shape that changed character depending on whether the two
    radii happened to be equal is worse than one that is uniformly a polygon.

15. **`SSHAPE` stores a handle in the string, not the pixels.** A real C128 packs the
    region's bitmap into the string variable. Here a value's string is a fixed 256 bytes
    (§1.2) and a saved region is a device surface, so the variable gets `SHAPE:<n>` and the
    surface stays in a fixed pool on the backend. `GSHAPE` accepts only a string carrying that
    prefix, so a hand-written one is refused rather than pasting an unrelated slot. **What
    this costs:** a shape cannot be written to disk, cannot be concatenated, and `LEN` of it
    is not its size. Nothing in BASIC does any of those to a shape string except a program
    deliberately poking at it.

16. **`BOX` cannot fill at all, and the plan for it is to spell fill as a negative angle.**
    7.0's last argument selects outline or fill and sits *after* the rotation, which would
    make a filled box a seven-argument call — one more than `akbasic_cmd_box()` collects.
    Spelling it as a negative rotation instead is the intended answer and **is not
    implemented**: today a rotation of zero outlines through the renderer's rectangle and any
    other rotation, negative included, draws four lines.

    **This entry used to claim in bold that it fills**, with the body then saying it was
    filed rather than implemented — a headline that contradicted its own paragraph, which is
    exactly how a reader ends up believing a feature exists. Found by writing a
    documentation figure for `BOX` and getting an outline back. `PAINT` is the fill a
    program has today.

    **`akbasic_GraphicsBackend::filled_rect` is dead from the language's side** as a direct
    consequence: `src/graphics_akgl.c` implements it and `tests/mockdevice.h` records it, but
    no BASIC verb reaches it. It is the entry point this fix would call, so it is waiting
    rather than unused — worth knowing before somebody tidies it away.

17. **`GRAPHIC` records its mode but honours only one consequence of it.** 7.0's five modes
    differ in bitmap resolution and in whether the bottom of the screen stays text. Neither
    means anything against a host's renderer, whose size the interpreter does not own. The
    mode is stored, an out-of-range one is refused because that is a typo worth catching, and
    the one observable rule is that mode 0 is text. The mode is readable as `RGR(0)`, which
    is 7.0's whole `RGR` and the one field of ours that is.

18. **A coordinate reaching a backend is a device pixel, and the whole of the host's window
    is reachable from BASIC.** This used to read "coordinates are BASIC's 320x200 space and
    scaling to the window is the host's job", on the reasoning that the interpreter cannot
    ask the renderer how big it is without owning it. **That reasoning was wrong twice over**,
    which is what §8 item 9 came to say.

    It was wrong about the mechanism: not owning a thing is no bar to *asking* it, and the
    backend record is how everything else here asks. `akbasic_GraphicsBackend` grew a `size`
    entry point, `require_graphics()` calls it before every verb that draws -- so a resized
    window is honoured between two statements rather than at attach -- and 320x200 became
    the fallback for a backend that leaves `size` NULL. It is the record's one optional
    entry point, so a host written against the old header keeps working and gets exactly the
    behaviour it used to get.

    And it was wrong about the description: nothing ever stretched anything. With `SCALE`
    off a coordinate was passed straight to `akgl_draw_*` as a pixel address, so an 800x600
    window drew a C128 listing into its top-left 320x200 corner and left the rest unused.
    The documentation described a coordinate transform that did not exist.

    **What changed observably** is `SCALE`, which now maps user coordinates onto the device
    rather than onto the constants, and `RGR(1)` / `RGR(2)`, which report the drawing
    surface's width and height so a program can use a window whose size it did not choose.
    A C128 listing draws in the corner as it always did; `SCALE 1, 319, 199` gives it the
    whole window.

    **One fix rode along, in the same line of code and too small to defer.** `SCALE` mapped
    `xmax` onto the *width*, which put the user space's far corner one pixel past the
    surface: `SCALE 1, 319, 199` then `DRAW 1, 319, 199` drew nothing at all. It maps onto
    the last pixel now, which is what makes that sentence above true rather than nearly
    true. `tests/graphics_verbs.c` and `tests/akgl_backends.c`, the latter against a
    128x128 target chosen because it is *smaller* than the old constants -- a `SCALE` still
    dividing by them misses it entirely rather than landing somewhere plausible.

19. **`PLAY` does not block.** On a C128 `PLAY` holds the program until the last note ends.
    §1.6 forbids that outright, so `PLAY` parses the string into a fixed queue and returns;
    `akbasic_runtime_step()` releases one note at a time against whatever time the host last
    passed to `akbasic_runtime_settime()`. **What this changes for a program:** the statement
    after a `PLAY` runs immediately rather than a bar later, so a listing that relied on `PLAY`
    for timing will race ahead. A program that wants to wait should test the queue rather than
    assume the verb waits for it.

20. **The host owns the clock, and an unset clock rushes the music.** The library reads no
    clock — it owns no loop and must not block. A host calls `akbasic_runtime_settime()` once a
    frame; the standalone driver calls it every step off `CLOCK_MONOTONIC`. Left unset it reads
    zero forever, so every duration has already expired and the queue drains as fast as
    `step()` is called. Audible, deterministic, and never a hang, which is the right way for
    that to fail.

21. **`PLAY`'s `M` (measure) is a no-op, and `SOUND`'s sweep runs once rather than
    oscillating.** `M` synchronises the three voices on a C128; with one sequential queue there
    is nothing to synchronise, and a listing full of them should still play, so it is accepted
    and ignored.

    The sweep *used* to be refused outright — there was no `akgl_audio_*` equivalent and faking
    one would have tied audible pitch to how often the host calls us. `libakgl` 0.3.0 added
    `akgl_audio_sweep()` and `SOUND voice, freq, dur, dir, min, step` now reaches it. What does
    not survive is `dir` 3, "oscillate": `akgl_audio_sweep` runs one pass between two endpoints,
    and a real oscillation needs the mixer to turn around at them. **What this changes for a
    program:** a `dir` 3 siren rises or falls once instead of warbling. `dir` 1 and 2 are exact,
    and so is the direction — both the SID and `akgl_audio_sweep` take it from which endpoint is
    higher, so a `min` above the starting frequency rises whatever `dir` says.

    `step` is converted as a *delta* rather than a position: it is a register increment, and the
    register-to-hertz table maps positions, so it goes across as the distance between register 0
    and register `step`. A step that converts to zero would never arrive and is raised to one
    hertz, which is the smallest move that still gets there.

    A backend whose record has no `sweep` — a host written against `libakgl` 0.2.0 — still
    refuses a swept note with `AKBASIC_ERR_DEVICE` and plays a held one normally.
    `tests/audio_verbs.c` asserts both halves.

22. **`TEMPO`'s constant is a calibration choice, not a transcription.** BASIC 7.0 documents
    `TEMPO` as a relative duration from 1 to 255 defaulting to 8, and does not publish what a
    whole note actually lasts. `src/audio_tables.c` uses 16000 ms at `TEMPO` 1, which puts a
    default-tempo quarter note at 500 ms — 120 beats per minute. If the real constant turns
    up, that is the one line to change.

23. **`GETKEY` holds the step loop rather than blocking.** A C128 sits on the keyboard inside
    `GETKEY` until somebody types. §1.6 forbids the library blocking, so the verb sets a flag
    and `akbasic_runtime_step()` declines to advance the program until a key arrives. Every
    step still returns and a bounded `akbasic_runtime_run()` still comes back, so a host keeps
    its frame rate; the program simply does not move past the `GETKEY`, which is the part a
    program author cares about. The `PLAY` queue is serviced *before* this check on purpose —
    music should keep playing while a program waits for a keypress. Withdrawing the input
    device while a `GETKEY` is holding releases it rather than wedging the script forever.

24. **`GET` and `GETKEY` into a numeric variable yield the key code.** BASIC 7.0's `GET` takes
    a string variable. Accepting an integer one as well, and giving it the raw code, is what a
    program testing for cursor or function keys needs and costs nothing. No key is code zero,
    matching what a C128 reports. A float variable is refused: it is neither a character nor a
    code.

### Deviations in the standalone frontend

Items 1–12 are deviations from ported interpreter code and 13–24 from BASIC 7.0. These four are
deviations from the reference's *program*: `main.go` and the SDL half of
`basicruntime_graphics.go`.

25. **The frame loop is bounded and the reference's is not.** Go's `run()` is a `for {}` that
    owns the process until `MODE_QUIT` (`basicruntime.go:682`), which is why the reference's
    window never answers its close button. Here the driver runs
    `AKBASIC_FRONTEND_STEPS_PER_FRAME` steps, pumps, presents, and goes round again. **What
    this changes for a program:** nothing observable — a bounded run reproduces an unbounded one
    exactly, and the whole golden corpus is driven through this loop to prove it. What it
    changes for a *user* is that the window closes when asked.

26. **The text layer repaints every row it owns, every frame.** Not just the rows that changed.

    That was tried — a `drawn[]` array marking which rows had carried glyphs, so an untouched
    row could be skipped — and it is **wrong on real hardware**. `SDL_RenderPresent` swaps
    buffers, so a frame does not inherit the frame before it; it inherits the one *two* back,
    and on some backends something undefined. A row erased once is clean in one buffer and still
    dirty in the other, and presenting alternates between them.

    That is what a backspace across a line wrap looked like: the first character of the wrapped
    tail flickering in and out forever after the rest had gone. It reproduces on X11 and never
    under the dummy driver or a software renderer, which is exactly why the suite was green
    through two rounds of fixing it. `tests/akgl_frontend.c` now paints stale pixels by hand —
    which is what a swapped-in buffer hands back — so the case is covered without needing real
    hardware.

    There is no cheaper correct answer available: a frame either owns every pixel it presents or
    it inherits pixels it cannot reason about.

    **What this costs, and it is more than it used to.** The host still does not call
    `SDL_RenderClear`, but the text area is now repainted in full every frame, so anything a
    graphics verb drew underneath it is erased every frame rather than only where text lands. In
    the standalone driver the text area is the whole window, so `DRAW` and `PRINT` no longer
    coexist there.

    **And the same buffer swap means the graphics verbs were never reliable across frames
    anyway**, which is worth stating plainly rather than leaving as a surprise: a one-shot `DRAW`
    lands in one buffer and the next present shows the other. It looked fine in a single
    screenshot and would have flickered exactly as the text did. Making graphics survive needs a
    persistent surface the frame is composited from, which is real work and its own commit —
    filed here rather than half-done.

27. **The cursor is a blinking block.** The reference draws an underscore glyph (`drawCursor`,
    `basicruntime_graphics.go:33`), and so did this until the underscore turned out to sit
    *under* the text being typed and make it hard to read. A block occupies the cell after the
    text instead of the space beneath it.

    Half a second per blink (`AKBASIC_SINK_CURSOR_BLINK_MS`), which is about a Commodore's and
    slow enough to read under. `akbasic_AkglSink.cursorperiodms` set to zero holds it solid,
    which is what makes a frame deterministic for a test.

    Two details that are decisions rather than accidents. The clock is **SDL's**, not the
    host's: everywhere else the host owns the clock because the library owns no loop and must
    not block, but this is an adaptor that already links SDL and threading a timestamp through
    the sink interface to animate a cursor would be a poor trade. And a line that exactly fills
    a row leaves the cursor one column past the end, because `putchar_at` wraps only when the
    next character arrives — the cursor is drawn at the start of the next row instead, which is
    where the next character will actually land and where a C128 puts it.

28. **The window is closed by the host, and that is not `QUIT`.** Closing the window stops the
    frontend and leaves `obj->mode` alone; it does not set `AKBASIC_MODE_QUIT`. The distinction
    is invisible to the standalone program, which exits either way, and load-bearing for an
    embedding host: a game that closes its own window has not decided that the script is
    finished, and a script that ran `QUIT` has. `tests/akgl_frontend.c` asserts both halves.

29. **Piped input still works in an AKGL build.** With a terminal on stdin the window is the
    console and lines come from its editor; piped or redirected, they come from the pipe. This
    is not in the reference, which always reads `os.Stdin` in REPL mode. It exists so
    `basic < program.bas` behaves the same in both builds — and so the golden corpus can be
    driven through the SDL binary, which is where most of the frontend's real coverage comes
    from.

### Deviations that change what a program is allowed to do

30. **A verb name with a type suffix is not a variable name.** `PRINT$ = 1` is refused where
    the reference accepts it, because its reserved-word check searched the keyword tables with
    the suffix still attached and therefore never matched (§6 item 16). A real C128 refuses it
    too, so this is the reference being wrong rather than this interpreter being strict.

    **What it costs a program:** a listing that used `INPUT$`, `LEN#` or `GOTO%` as a variable
    stops parsing, and the fix is to rename the variable. One case in the reference's own corpus
    did exactly that; see `tests/language/examples/strreverse.bas`.

### Deviations in statement separation

31. **A branch decides who owns the rest of its line.** BASIC 7.0 scopes every statement after
    `THEN` to the condition, and the reference has no opinion on the matter because it never
    consumed the `COLON` token at all. The parser here takes exactly one statement for each arm,
    so the rest of the line arrives at the statement loop as ordinary top-level statements and
    something has to say whether to run them. The `BRANCH` case in
    `akbasic_runtime_evaluate()` sets `runtime->skiprestofline`, and the two statement loops
    stop on it.

    **The rule is not "skip when false"**, which is the reason this is written down:

    | Line | Condition | What runs |
    |---|---|---|
    | `IF C THEN A : B` | true | `A`, `B` |
    | `IF C THEN A : B` | false | nothing |
    | `IF C THEN A ELSE B : D` | true | `A` |
    | `IF C THEN A ELSE B : D` | false | `B`, `D` |

    The remainder always belongs to whichever arm was written *last*, so it is skipped exactly
    when that arm is the one not taken — with an `ELSE` the last arm is `ELSE`, without one it
    is `THEN`. That is one line of code and it reads as an oddity without the table above it.

### Deviations in the structure and error verbs (groups A and C)

61. **Structures are an addition, not a port.** BASIC 7.0 has no records at all, so
    `TYPE`/`END TYPE`, `DIM X@ AS T`, `.` and `->`, `PTR TO` and `POINT ... AT` are
    invented here. The `@` suffix was not: the Go reference reserved
    `IDENTIFIER_STRUCT` and never used it, and `src/grammar.c` rendered such a leaf as
    `"NOT IMPLEMENTED"` until this landed. `docs/16-structures.md` is the whole feature.

    **Assignment copies and `POINT` shares**, which is the decision everything else rests
    on. A structure is a value like every other value here, so a program that never writes
    `POINT` can never be surprised by aliasing — and the two field operators are kept
    apart so a reader always knows from the spelling which they are looking at. `.` on a
    pointer and `->` on a value are both errors, each naming the other.

    **A declared type is what buys the storage model.** An instance has a known slot count
    and is laid out exactly as an array is, out of the same value pool, so nothing new
    holds data — only a table of descriptors. It also bounds copy depth statically, since a
    `TYPE` cannot contain itself by value.

    **Three limits are ours and are stated rather than derived:** 16 types, 16 fields per
    type, and four levels of nesting in `PRINT`. The last is not a safety bound on copying —
    copy stops at pointers by construction — but on *rendering*, which must follow a
    pointer and would not come back on a cycle. Four rather than eight because the bound
    has to bite before the 256-byte render buffer does, or a cycle stops because it ran out
    of room rather than because it was told to.

    **A type name shares a namespace with verbs and labels**, since all three are bare
    words. Refused at declaration with a message that says so, because the parser's own
    answer was "Expected expression or literal" pointing at the line rather than the
    problem. Field names follow the same reserved-word rule variable names already do,
    enforced by the loader's scan — `TO@` is a bad field name for exactly the reason `TO#`
    is a bad variable name.

62. **A host's C struct is the same thing with its bytes somewhere else.**
    `akbasic_host_register_type()` puts it in the *same* table a `TYPE` fills, so copy,
    `PTR TO`, `.` and `->` all work across the boundary with no second set of rules — and
    the language's own copy-versus-`POINT` distinction turns out to be exactly the
    snapshot-versus-share distinction a host needs, so there is one API rather than two.

    A binding takes a **shadow run** of slots; a read refreshes from host memory and a
    write converts back. Conversion **refuses rather than truncates** — 70000 into an
    `int16_t` names the field — and a field name's suffix must agree with the C type it
    describes, refused at registration.

    **It is the only pointer this interpreter holds that it did not allocate**, and that
    is the one thing a host has to think about. `akbasic_host_unbind()` exists for it.
    `examples/hoststruct.c` is a working host, built and run by every build.

44. **A whole loop on one line does not loop.** `DO : PRINT 1 : LOOP` runs once, exactly as
    `FOR I=1 TO 3 : PRINT I : NEXT I` does. Block skipping walks *source lines*, which is the
    reference's `waitingForCommand` model (§1.6), so a `LOOP` on the same line as its `DO` is
    never reached as a separate step. Fixing it means making the skip operate on statements
    rather than lines — a deliberate piece of work, already noted in §4 against `FOR`.

45. **`ER` and `EL` are `ER#` and `EL#`.** A C128 exposes them as bare reserved names. This
    dialect cannot: an identifier carries its type in a suffix, and a name without one is a
    *label*. They are written into the global scope when a trap fires, so `PRINT ER#` reads the
    way the rest of the language does and needs no new syntax.

    `ER#` carries the `libakerror` status code, not a Commodore error number — 515 for an
    out-of-bounds subscript rather than C128 error 9. There is no correspondence to reproduce:
    the errors this interpreter raises are not the errors a 1985 ROM raised. `ERR(ER#)` gives
    the name, and that is the part a program should be printing anyway.

46. **`END` does not arm `CONT`.** A C128 lets `CONT` resume after `END` as well as after
    `STOP`. Here `END` means the program finished, and continuing from a finished program
    resumes at whatever line happens to follow — so `CONT` refuses instead. `STOP` still arms it.

### Deviations in the format verbs (group D)

47. **`PRINT USING` renders one field per statement.** BASIC 7.0 lets a format string hold
    several fields and `PRINT USING` take a list of values against them. Here the *first* field
    is used and any literal text around it is copied through, so `PRINT USING "A: ###"; X` works
    and `PRINT USING "### ###"; X, Y` does not. Multiple fields need `PRINT` to take a value
    list, which it does not yet — a separate piece of work from the field rendering itself,
    which `src/format.c` does properly.

    Exponential fields (`^^^^`) are not implemented either. Nothing in the language produces a
    value that needs one, since `PRINT` renders a float with `%f`.

48. **`WIDTH` is emulated with parallel passes.** It sets the thickness of drawn lines, and
    neither the graphics backend record nor `akgl_draw_line` takes a thickness — so a `WIDTH 2`
    line is drawn twice, offset one pixel along whichever axis the line does *not* mostly run
    along. Visually right at the only other value the verb accepts; a general thickness would
    want a real perpendicular offset and an API that takes one.

49. **`CHAR` ignores its colour argument, and needs a sink that has a cursor.** The text sink
    draws in one colour, chosen by the host when it built the sink, so a per-call colour would
    have to become part of the sink interface. The argument is accepted and ignored rather than
    refused, because refusing it would make every published `CHAR` line a syntax error.

    Positioning is a new optional `moveto` on the sink record. A stdio sink leaves it NULL and
    `CHAR` refuses by name — a terminal's cursor is not this library's to move and a pipe has no
    cursor at all — so `CHAR` works in an AKGL build and says why it cannot in a stdio one.

### Deviations in the console verbs (group E)

50. **`SLEEP` and `WAIT` hold the step loop; they do not block.** §1.6 forbids blocking, so a
    waiting program is one that does not *advance* — `akbasic_runtime_step()` still returns, a
    bounded `akbasic_runtime_run()` still comes back on time, and an embedded game keeps its
    frame rate while a script sleeps.

    `SLEEP` with no host clock does nothing at all rather than waiting forever. A deadline
    computed from a clock that never advances is never reached, and the first version of this
    hung the test suite proving it.

51. **`TI` and `TI$` are `TI#` and `TI$`.** Same reason as `ER#`/`EL#`: no bare variable names.
    They are ordinary globals refreshed once per step from the host's clock, rather than
    pseudo-variables, because this dialect has no mechanism for a name that computes itself.
    A host that never sets a clock gets a stopped clock rather than a wrong one.

52. **`WAIT` polls ordinary process memory.** On a C128 the byte is a hardware register an
    interrupt is changing. Here nothing changes it but the host, another thread, or a `POKE`
    from a `TRAP` handler — so a program that waits on a byte nobody writes waits forever, which
    is exactly what the same program does on a C128 with the wrong address. Read through a
    `volatile` pointer, because the whole point is that something outside this program writes it.

53. **`KEY` stores its macros and nothing expands them.** The definitions are kept and bare
    `KEY` lists them, which is the half that is this library's business. Expanding a macro when
    a function key is pressed belongs to whatever owns the keyboard — the frontend's line editor
    — and the input backend delivers keycodes rather than editor commands.

### Deviations in the machine verbs (group J)

54. **`FETCH` and `STASH` are the same byte copy.** On a C128 they differ by which side is the
    RAM Expansion Unit — `STASH` writes out to it, `FETCH` reads back. There is no expansion
    unit and there are no banks, so both are `memmove` between two addresses and saying so is
    better than inventing a distinction. `memmove` rather than `memcpy` because a program
    shifting a buffer along by a few bytes overlaps its own source, which is a normal ask.

    Addresses are real process addresses, which is the decision `POKE`, `PEEK` and `POINTER`
    already made. There is no bounds check and there cannot be one: a wrong address is a
    segmentation fault, not an error message.

55. **`SYS` is refused by name.** It calls machine code at an address. There is no 6502 and no
    ROM to call, so there is nothing to jump to and nothing sensible to emulate — and jumping to
    a real address in this process would be a way to crash it on purpose. It parses first and
    refuses second, so a listing containing `SYS` still loads and still lists; only running it
    fails, and it says why.

    Same reasoning as `BANK`, `FAST` and `MONITOR`, with one difference: those have no table row
    at all, and `SYS` has a handler because it is common enough in published listings to deserve
    a specific message rather than "Unknown command".

### Deviations in the disk verbs (group F)

56. **There is no 1541, so the verbs split three ways.** The ones that mean something on a
    filesystem are implemented against `aksl_f*` — `DOPEN`, `DCLOSE`, `APPEND`, `RECORD`,
    `SCRATCH`, `RENAME`, `COPY`, `CONCAT`, `BSAVE`, `BLOAD`, `VERIFY`. The ones that are
    spellings of verbs already here are aliases: `SAVE` is `DSAVE`, `LOAD` is `DLOAD`, `CATALOG`
    is `DIRECTORY`, `DVERIFY` is `VERIFY`.

    **`HEADER`, `COLLECT`, `BACKUP` and `BOOT` are refused by name**, with the reason. Each
    operates on a physical disk — formatting one, validating its block allocation map,
    duplicating it, booting from it — and a filesystem has no equivalent that is not a lie.
    `HEADER` would have to mean "delete everything in this directory", which is a
    spectacularly bad thing to do to somebody who typed a C128 verb. `DCLEAR` is the exception:
    resetting a drive also closes its channels, and closing the channels is real, so that is
    what it does.

57. **`DIRECTORY` is refused for want of an upstream wrapper.** `libakstdlib` has no
    `aksl_opendir`, and this project's rule is that a missing capability is filed upstream
    rather than worked around — so it is filed against `libakstdlib` and the verb says
    so. Calling `opendir(3)` here would mean reporting through `errno` in a file where
    everything else reports through an `akerr_ErrorContext *`.

58. **`PRINT #` and `INPUT #` need the space before the `#`.** A C128 writes `PRINT#1,A$`. The
    scanner reads `#` as a type suffix, so `PRINT#` comes out as an identifier named `PRINT`
    with an integer suffix — and a verb name carrying a suffix is refused (deviation 30). With
    a space the `#` is its own token and the parse handler reads it.

59. **`RECORD` counts lines, not fixed-length records.** A C128's relative file has a record
    length fixed when the file was created; a filesystem file has none. So a record is a line
    and `RECORD` rewinds and reads forward to it, which is slower than a seek and is the only
    definition that does not invent a record length the file does not have.

60. **`BLOAD` requires a length.** A C128 reads until the file ends. Here the address is a real
    process address, so a file longer than the caller expected would write past whatever it was
    pointed at with no way to notice. Refusing to guess is the only safe answer.

### Deviations in conditions

41. **A lone `=` is equality inside a condition.** `IF A# = 2 THEN` works, which it did not
    before: the scanner reads `==` as EQUAL and a single `=` as ASSIGNMENT, because at that
    point it cannot know whether it is looking at a statement or a condition, and the reference
    never resolved the ambiguity anywhere. A condition is the one context where it has an
    answer — BASIC has no assignment expression — so `akbasic_Parser::comparing` is set around
    the condition and the relation parser offers ASSIGNMENT as a comparison operator only while
    it is. `==` keeps working, and the whole checked-in corpus is written in it.

    Outside a condition `=` has to stay an assignment, which is what the flag is for: the first
    attempt put ASSIGNMENT in the operator list unconditionally, and `FOR I# = 1 TO 5` promptly
    stopped initializing its counter.

42. **A condition is a whole expression, so `AND` and `OR` work in one.**
    `IF A# = 5 AND B# = 3 THEN` used to report "Incomplete IF statement". The reference parses a
    single *relation* after `IF`, and a relation sits below `AND` and `OR` in the grammar chain,
    so the `AND` was never consumed and the `THEN` check found the wrong token.
    `akbasic_parse_if()` calls `akbasic_parser_expression()` now.

43. **Truth is nonzero, not "a boolean holding true".** `akbasic_value_is_truthy()` is what a
    branch tests. Commodore BASIC has no boolean type: a comparison yields -1 or 0, `AND` and
    `OR` *are* the bitwise operators, and `IF A THEN` is legal for any numeric A. The reference
    tests `boolvalue == -1` regardless of the value's type, so `IF A# = 1 OR B# = 2 THEN` — whose
    `OR` yields an integer — was silently always false, and so was `IF A# THEN`.

    The bitwise operators accept a truth value as an operand for the same reason: -1 is every
    bit set precisely so that `AND` and `OR` double as the logical pair. A string is still never
    true; a C128 raises a type mismatch there, which is a stricter answer this could adopt later.

### Deviations in the sprite verbs (group H)

32. **Labels are filed before the program runs, not as each `LABEL` executes.**
    `akbasic_runtime_scan_labels()` walks the stored source on every entry into
    `AKBASIC_MODE_RUN` and files every `LABEL <name>` it finds, textually, without parsing the
    lines around it. The reference resolves a label only once its `LABEL` statement has run, so
    `GOTO` and `GOSUB` reached backwards and never forwards.

    **This was not a nice-to-have.** An interrupt handler by definition sits on a line normal
    flow does not fall into, so `COLLISION 1, BUMPED` could not be made to work at all under the
    old rule — neither resolving at arm time nor at fire time helps when the `LABEL` line is
    never executed. Forward `GOTO` is the improvement that came with it, covered by
    `tests/language/statements/label_forward.bas`.

    `LABEL` still executes and still files itself, so a name that appears twice resolves to the
    last one in the source until one of them runs. The scan is textual on purpose: parsing every
    line up front would raise on lines the program would never have reached.

33. **Sprite coordinates are device pixels, not the VIC-II's 0–511 by 0–255.** `MOVSPR` takes
    the same coordinate space `DRAW` does. A C128's sprite coordinates are the raster's, offset
    so that (24, 50) is the top-left of the visible screen; reproducing that would give the
    language two coordinate systems and make `MOVSPR 1, 0, 0` put a sprite off-screen. Consistent
    with deviation 18, which is where "the same space `DRAW` uses" is defined — and it moved,
    so this one moved with it.

    **`SCALE` deliberately does not apply to sprites.** It is a graphics-verb transform and
    the sprite verbs do not call it, which was true before deviation 18 was rewritten and is
    worth writing down now that the two spaces can differ: with `SCALE 1, 319, 199` on, a
    `DRAW` at 160,100 and a `MOVSPR` to 160,100 land in different places. Arguably they
    should agree; not changed here because it is a decision about what `SCALE` means rather
    than a slip, and it wants its own commit.

34. **`MOVSPR`'s speed unit is ours.** BASIC 7.0 documents speed 0–15 with 15 fastest and says
    nothing about what a unit is worth. Here one unit is
    `AKBASIC_SPRITE_SPEED_PIXELS_PER_SECOND` (5) pixels per second, so speed 15 crosses the
    320-pixel screen in about four seconds. Paced off `akbasic_runtime_settime()` from
    `akbasic_runtime_step()`, exactly as the `PLAY` queue is, so a host that never sets a clock
    gets a still picture rather than a hang — same trade as deviation 20.

35. **`SPRSAV` takes an integer array where a C128 takes a string, and takes a file path as
    well.** Three source forms, one of which is the C128's:

    | Form | Source |
    |---|---|
    | `SPRSAV A$, n` | a region `SSHAPE` saved. The C128 documents `SPRSAV`'s string as *the `SSHAPE` data format at a fixed 24×21*, so sharing the shape pool is faithful rather than a shortcut |
    | `SPRSAV A#, n` | 63 elements of a DIMmed integer array — the DATA-driven path a type-in listing uses |
    | `SPRSAV "ship.png", n` | an image file |

    **The array form exists because a string here cannot hold a sprite.** A value carries a
    NUL-terminated `char[256]` (§1.2) and a pattern is 63 raw bytes including zeros, so the
    C128's own spelling is unrepresentable. `DIM P#(63)` is exactly the right size.

    **The file form is a deliberate addition, not a fallback.** A string source is a path unless
    it starts with `SHAPE:`, which is the prefix `SSHAPE` mints, so the two never collide. It
    resolves through `akgl_path_relative()` — the working directory first, then the directory the
    running program was loaded from — and loads through `akgl_spritesheet_initialize()`, which
    are the same two calls `akgl_sprite_load_json()` makes for a spritesheet. A sprite loaded
    this way takes **the image's own size** rather than being forced to 24×21; a modern PC has
    no reason to throw away art that is not sprite-shaped.

    `akbasic_runtime_set_source_path()` is what tells the interpreter where the program came
    from. Group F's disk verbs will want the same thing.

    **The reverse direction is refused.** `SPRSAV n, A$` on a C128 copies a sprite *out*; here
    that would mean writing an image file, which is a disk operation, and group F is
    unimplemented as a whole. Refused by name rather than half-built.

36. **`COLLISION` implements type 1 only, and types 2 and 3 are refused by name.**
    Sprite-to-background needs the whole render target read back and compared against each
    sprite every frame; a light pen has no meaning on a machine with no light pen. Both raise
    `AKBASIC_ERR_DEVICE` with the reason rather than being accepted and never firing.

    Collision is **bounding-box, not pixel**. A C128's VIC-II collides on set pixels, so two
    sprites whose boxes overlap but whose art does not are reported as colliding here and would
    not be there. Pixel-exact collision would mean keeping every sprite's unpacked bitmap and
    testing the overlap rectangle a pixel at a time; the box test is four comparisons.
    `akgl_collide_rectangles()` is deliberately not used — it has a documented
    corner-containment defect that misses a plus-shaped overlap, and nothing in libakgl calls it.

37. **A collision handler is entered between source lines, and must end in `RETURN`.**
    `akbasic_runtime_service_interrupts()` runs at the top of `akbasic_runtime_step()` and
    injects what amounts to a `GOSUB` the program did not write. Between lines is the only safe
    place: a handler entered mid-statement would have to return into the middle of a line and
    the parser keeps no state that could resume there. That is the same granularity block
    skipping already works at (§1.6).

    An interrupt does not interrupt an interrupt. A collision that is still true while its own
    handler runs would otherwise re-enter on the next line until the environment pool was gone.
    The event is not lost — it is taken as soon as the handler's `RETURN` lands.

38. **Sprite priority is recorded and not honoured.** `SPRITE n,,,1` sets the bit, `RSPRITE`
    reads it back, and nothing draws differently. A C128 draws a low-priority sprite behind the
    bitmap plane; here the text layer, the drawing surface and the sprites share one render
    target and sprites are composited on top of it every frame, so there is nothing to go
    behind. Honouring it would mean a separate composited surface per plane — which is the same
    piece of work deviation 19 already needs.

39. **Multicolour mode is recorded and not honoured.** Same shape as priority: `SPRITE`'s
    seventh argument and `SPRCOLOR`'s two registers are kept, `RSPRITE` and `RSPCOLOR` read them
    back, and no sprite is drawn in multicolour. Multicolour packs *two* bitmap bits per pixel
    and selects between the sprite's own colour and the two shared registers; every `SPRSAV`
    source form here carries one bit per pixel or a full-colour image, so there is no second bit
    to select with. The argument positions are kept honest for the day a multicolour pattern
    format exists.

40. **A sprite is a real `libakgl` actor, and it is drawn by a renderfunc of ours.** Each of the
    eight becomes a `SpriteSheet` + `Sprite` + `Character` + `Actor` registered in
    `AKGL_REGISTRY_ACTOR`, so an embedding game sees BASIC's sprites alongside its own (goal 3).
    But `akgl_actor_render()` is not what draws them. There were two reasons and **libakgl
    0.5.0 fixed one**: the default used to compute its destination *height* from the sprite's
    **width**, drawing a 24×21 Commodore sprite as a 24×24 square, and it now uses the height
    — libakgl defect 26, filed from here and closed there.

    **The reason that remains is the one that keeps this file.** An actor carries a single
    scalar `scale` applied to both axes, which cannot express `SPRITE`'s separate x- and
    y-expand bits, so `MOVSPR`'s twice-as-wide-same-height is unrepresentable. libakgl records
    it as open and names this interpreter as the caller that needs it; the fix is a design
    choice over there — `scale_x`/`scale_y` beside `scale`, or replacing `scale` and taking
    the ABI break while the major is still 0 — rather than a patch. Until then
    `src/sprite_akgl.c` installs its own `renderfunc`, which is libakgl's own extension point
    for exactly this.

    Worth keeping as a pattern: both defects were found by *using* the library for something
    it had not been used for, and both were filed rather than worked around silently. One came
    back fixed.

63. **Line numbers are optional in a loaded program, and a blank line is not a program line.**
    **This one moved a golden file.** A line with no number used to be filed under the loader's
    cursor unchanged — that is, on top of the line before it — so two unnumbered lines in a row
    silently lost the first, and a *blank* line erased whatever preceded it. The reference does
    the same, and `tests/language/arithmetic/integer.bas` is the proof: four `PRINT`
    statements, an expectation with three values, and a trailing blank line that erased
    `40 PRINT 4 - 2` before the program ran. The expectation is now `4 4 2 2` and
    `tests/language/README.md` records it.

    In its place: `akbasic_runtime_file_line()` (`src/runtime.c`) is the one implementation of
    the rule, shared by `akbasic_runtime_load()`, RUNSTREAM and `DLOAD`. A numbered line is
    filed under its number and moves the cursor; an unnumbered one takes the slot after it;
    blank lines are skipped by all three. A collision involving an assigned number is refused
    with `AKBASIC_ERR_BOUNDS` rather than overwriting. `akbasic_SourceLine` grew a `numbered`
    flag so deviation 64 can tell the two apart.

    **The prompt is deliberately untouched.** A line typed without a number is direct mode and
    runs now; that is the only thing separating program text from a statement at a REPL, and it
    is why this is a *loading* feature rather than a language-wide one. `AUTO` remains the way
    to enter program text without typing numbers.

    Consequence worth stating: an unnumbered program is capped at 9998 lines, the cap that
    already existed, and its assigned numbers are increment-1 — so `LIST` and `DSAVE` show a
    listing with no gaps to insert into. `RENUMBER` before `DSAVE` gives the gaps back, and
    marks every line numbered.

    **Not changed:** two lines that both carry the *same* written number still silently keep
    the last, as they always have. That is a separate decision with its own corpus risk and it
    is not this one.

64. **Branching by number to a line the program did not number is refused before it runs.**
    The other half of deviation 63, and the reason `akbasic_SourceLine` carries a flag rather
    than the loader just picking slots quietly. In a script written without line numbers
    `GOTO 100` finds the hundredth line and branches there: plausible, silent and wrong, which
    is the worst thing to hand somebody at run time.

    `akbasic_runtime_check_targets()` is a fourth prescan, run beside the label, `DATA` and
    `TYPE` ones on every entry into `AKBASIC_MODE_RUN` — the earliest the check can be made and
    the only place all four ways a program arrives pass through. It refuses with
    `AKBASIC_ERR_SYNTAX`, naming the line and saying what to do instead:

    ```text
    ? 1 : PARSE ERROR Line 1: branch to line 4, which the program did not number.
          Branch by LABEL, or RENUMBER first
    ```

    Two things are deliberately **not** refused. A target naming an *empty* line, for the same
    reason `RENUMBER` leaves one alone — `GOTO 9999` in a program with no line 9999 is already
    broken and inventing a rule about it would hide that. And a target in a fully numbered
    program, which is every program that existed before this, so nothing checked in changed.

    **It shares `RENUMBER`'s walk rather than repeating it.** `src/renumber.c` grew an
    `akbasic_TargetWalk` — a `self` pointer and a `visit` function — and `rewrite_line()` now
    takes one. `RENUMBER`'s visitor substitutes the number a line moved to; the check's
    substitutes the number unchanged and raises if it names an assigned line. One walk, so the
    two cannot disagree about what a branch target is, which they would have within a release.

    Worth knowing: the check points `environment->lineno` at the line it is walking, so the
    `? N :` prefix names the offending line. **The other three prescans do not**, so a
    malformed `TYPE` or a bad `DATA` item still reports whichever line the loader stopped on —
    usually the last line of the program. They work around it by putting `Line %d:` in the
    message text, which is why the message above says the number twice. Worth fixing in
    `akbasic_runtime_set_mode()`'s wrapper rather than in four places; not fixed here.

---

---

## 6. Reference defects — **fix them; fidelity is no longer a reason not to**

These are real bugs in `deps/basicinterpret`, reproduced faithfully during the port.

**The rule that governed this section is gone.** It used to read "port the behaviour first so
the golden suite passes and the port is provably faithful, then fix each one deliberately" —
and faithfulness was the reason several of these are still here. The Go implementation is
deprecated and will not be updated (§0.1), so there is no longer anything to be faithful *to*.
Each of these is now an ordinary defect, to be judged on whether the fix is right rather than on
whether it matches.

What has not changed is the working method: one fix per commit, with a test that asserts the
correct contract, and a golden file moved in the same commit if the fix changes output.
`AKBASIC_KNOWN_FAILING_TESTS` is empty now and is kept declared for the next defect that has to
be reproduced before it can be fixed.

1. ~~**`basicvariable.go:176`** — `toString()` tests `len(self.values) == 0` and then indexes
   `self.values[0]`.~~ **Never ported.** There is no `akbasic_variable_to_string`: rendering a
   value is `akbasic_value_to_string()`, which switches on the value type and has no count test
   to invert. The buggy function had no counterpart to reproduce.
2. ~~**`basicvariable.go:108`** — `setBoolean` builds a value with `valuetype: TYPE_STRING`.~~
   **Fixed in the port.** `akbasic_value_set_bool()` sets `AKBASIC_TYPE_BOOLEAN`, and
   `tests/value_compare.c` reads a boolean back through every comparison operator.
3. ~~**`basicenvironment.go:157`** — `stopWaiting(command)` ignores `command`.~~ **Fixed in
   the port.** `akbasic_environment_stop_waiting()` walks the scope chain and clears the first
   environment actually waiting for *that* verb, so an inner block can no longer clear an outer
   block's wait. A miss is tolerated rather than raised, which is the one part of the suggested
   fix not taken: `EXIT` and the end of a `DEF` body both call it speculatively.
4. ~~**`basicvalue.go:181`** — `mathPlus` mutates `self` in place when `self.mutable` is
   true.~~ **Fixed.** It clones like every other operator now, so `A# + 1` cannot modify `A#`.

   The gate on this item was `FOR`/`NEXT` coverage, because `NEXT`'s increment *relied* on the
   mutation to advance the counter. `tests/for_next.c` is that coverage -- `STEP`, a negative
   step, a float counter, a body that assigns to the counter, `EXIT`, and nesting, none of which
   the golden corpus reaches -- and `akbasic_cmd_next()` now writes the incremented value back
   with `akbasic_variable_set_subscript()`. Removing that write-back does not fail the suite, it
   *hangs* it: the counter stops advancing and the loop never ends.
5. ~~**`basicvalue.go:191` and siblings** — every binary operator adds both of the right-hand
   operand's numeric fields.~~ **Fixed.** `rval_as_int()` and `rval_as_float()` select on the
   operand's type instead of summing `intval + (int64_t)floatval`.

   It was filed as a landmine with no consequence today, and that is why it needed a test
   written against the value API rather than against the language: no BASIC program can build a
   value carrying both fields, and the old code passes every other test in the tree.
   `tests/value_arithmetic.c` constructs one directly.
6. ~~**`basicvalue.go`, all comparisons** — `dest` is cloned from `self`, so the resulting
   boolean inherits `self.name`.~~ **Moot.** `BasicValue.name` was dropped as dead during the
   port along with `BasicEnvironment.update()`, its only reader — deviation 9 above. There is no
   name for a comparison result to inherit.
7. ~~**`basicruntime_commands.go:484`** — `CommandIF` dereferences `expr.right` after the loop
   guaranteed it is `nil`, so the "Malformed IF statement" check is unreachable.~~ **Fixed in
   the port.** `akbasic_parse_if()` matches the `THEN` token and compares its lexeme directly,
   so the check runs on every `IF`.
8. ~~**`basicruntime_commands.go:669`** — `CommandEXIT` pops the environment without
   `stopWaiting`.~~ **Fixed**, and then fixed again — see item 18, which is what clearing the
   wait left behind.
9. ~~**`basicruntime.go:149`** — `newVariable()` and `BasicRuntime.variables[MAX_VARIABLES]`
   are dead.~~ **Not ported as dead code.** The pair is live here and is the storage model:
   `akbasic_environment_create()` takes a slot from `akbasic_runtime_new_variable()`, because a
   pool is what replaced Go's per-environment map (§1.3). The reference's versions were dead;
   these are not.
10. ~~**`basicgrammar.go:224`** — `newLiteralInt` selects base 8 whenever the lexeme starts
    with `0`.~~ **Fixed.** Base 10 unless prefixed `0x`; a leading zero in a listing is padding,
    not a radix. `PRINT 010` prints 10 and `PRINT 08` parses. Nothing in the upstream corpus
    had a leading-zero literal, which is why nothing there noticed. Regression tests in
    `tests/grammar_leaves.c` and `tests/language/numeric/octal_literal.bas`, which was rewritten
    from pinning the defect to pinning the fix.
11. ~~**`basicruntime.go:121` / `basicparser_commands.go:124`** — environments are never
    freed.~~ **Fixed in the port.** They come from a fixed pool and
    `akbasic_runtime_prev_environment()` releases each one — deviation 3 above. An unreleased
    scope shows up as pool exhaustion rather than as unbounded growth, which is how item 18
    announced itself.
12. ~~**`basicparser.go:329` — `subtraction()` returns after one operator where `addition()`
    loops.**~~ **Fixed.** `1 - 2 - 3` computed `1 - 2` and abandoned the rest of the line; the
    remaining `- 3` was left in the token stream, picked up by the statement loop as a second
    statement, evaluated and discarded. A wrong answer, silently, which made this the
    highest-impact item on the list. `exponent()` has the identical shape — a `for` loop with a
    `return` inside it, in both the reference and the port — so `2 ^ 3 ^ 2` had it too. Both
    now fold left, which is what a C128 does with a run of same-precedence operators.
    Regression tests in `tests/parser_expressions.c`.

    Worth recording for whoever reads the reference next: `exponent()` there is
    `for self.match(CARAT) { ... return expr, nil }` (`basicparser.go:519-539`), so the loop is
    a loop in shape only. `subtraction()` is the same at `:357-377`. Neither is a
    transcription slip in the port.

13. ~~**`basicparser.go:565` — a unary-minus argument inflates a function's arity count.**~~
    **Fixed**, and the fix is structural rather than a smarter counter. The counter walked
    `.right`, which is where a unary leaf keeps its operand *and* where an argument list chains
    its arguments; the two meanings were indistinguishable, so `ABS(-9)` counted as two
    arguments and was refused with "function ABS takes 1 arguments, received 2". No builtin
    could be called with a negative literal at all. The corpus hid it — `sgn.bas` assigns `-1`
    to a variable first.

    **A unary leaf's operand now hangs off `.left`.** A unary leaf had no other use for that
    field, so the two meanings separate for the cost of one nobody was using — and counting
    then needs no special case at all. That also fixes the half of the defect the original
    entry did not mention: chaining the *next* argument through `.right` overwrote the operand,
    so `MOD(-7, 3)` destroyed its own first argument. Three readers moved (`src/runtime.c`,
    `src/grammar.c`'s renderer, `src/runtime_commands.c`'s `LIST` range parser) and that is the
    whole change. Regression tests in `tests/runtime_evaluate.c`, both halves.

    **The same collision remains for array subscripts**, which is why §4 still lists
    "array references in parameter lists" as outstanding: an identifier keeps its subscript
    list on `.right` too. The same move — onto `.left` — is very likely the fix, and it is not
    made here because it wants its own commit and its own tests.

14. ~~**`basicscanner.go:272` — `matchNextChar` returns without setting a token type when it
    cannot peek past the end of the line.**~~ **Fixed.** A comparison operator in the final
    column was silently dropped: `A# =` produced one token, not two, and the parse error that
    followed pointed somewhere else entirely. `falsetype` is now set before returning on the
    peek failure. Regression tests in `tests/scanner_tokens.c`, for `=`, `<` and `>`.

15. ~~**`basicscanner.go:318` — hex literals do not survive the scanner.**~~ **Fixed.**
    `matchNumber` let `x` through but not the hex digits after it, so `0xff` lexed as `0x`
    followed by an identifier `ff`, the base-16 branch in `newLiteralInt` was unreachable, and
    the README's hex support did not exist. Once the `x` is seen the run continues over hex
    digits. Accepted anywhere in a number run rather than only after a leading `0`, which is
    the reference's rule and is worth keeping: it is what makes `1x2` one malformed token that
    the line-number conversion diagnoses by name. Regression tests in `tests/scanner_tokens.c`
    and `tests/language/numeric/octal_literal.bas`.

16. ~~**`basicscanner.go:349` — the "Reserved word in variable name" check never fires.**~~
    **Fixed**, and it is the item §0.1 unblocked. The keyword tables are now searched on the
    *base* name with any type suffix stripped, so `PRINT$`, `LEN#` and `GOTO%` are refused as
    variable names and the reference's own diagnostic stops being dead code. A name that merely
    contains a verb — `PRINTER$` — is still fine, which is what says the check compares the
    whole base name rather than a prefix. Regression tests in `tests/scanner_tokens.c`.

    **It cost one golden case, exactly as predicted, and the cost turned out to be nothing.**
    The reference's `examples/strreverse.bas` names a variable `INPUT$`; the variable is renamed
    to `SOURCE$` in `tests/language/examples/strreverse.bas`, the expectation is byte-for-byte unchanged — the program
    still prints `REVERSED: OLLEH` — and the case keeps every bit of its coverage. Recorded in
    `tests/language/README.md` records the corpus editing rule.

    This item sat parked because the corpus was the acceptance contract and lived in a submodule
    this repository could not edit. Both premises are gone: the corpus is checked in, and
    matching the Go implementation is no longer a goal. Worth keeping the history, because it is
    a clean example of a fix that was correct all along and blocked entirely by a constraint
    that has since expired.

### Ours, not the reference's

17. ~~**A host cannot reliably create a script variable while a script is suspended.**~~
    **Fixed.** `akbasic_runtime_global(obj, name, dest)` walks `obj->environment` to the root
    and finds or creates there unconditionally. `README.md` and `examples/hostvars.c` both point
    at it now, and `tests/hostvars.c` asserts it: a global created from inside a `FOR` body and
    from inside a `GOSUB`, still readable by the script after the body pops, plus the
    seed-before / read-after path that always worked.

    This one was not inherited — it fell out of §1.4's environment pool meeting §1.6's bounded
    `run()`, a combination the reference never had because its `run()` never returned. Both
    failure modes were silent: a variable created through `akbasic_environment_get()` landed in
    the loop's scope and died with it, so the script read it correctly inside the loop and got
    `0` immediately after; and reaching for the root explicitly returned `NULL` through `dest`
    **without raising**, because that function only auto-creates when
    `obj->runtime->environment == obj`.

    Three things settled while doing it, each worth keeping:

    - **The design question §6 named has an answer.** Suspended inside a *user function's*
      scope, the root is still right and the walk still gets there: a funcdef's environment is
      initialized with the caller's as its parent (`akbasic_runtime_user_function`), so the
      chain terminates at the same root as any other.
    - **`akbasic_environment_create()` is the new half**, and it searches *only* the scope it
      is given — no walk in either direction. Walking up would find an outer variable and put
      the value somewhere other than where the caller asked for it.
    - **`akbasic_environment_get()` is unchanged.** Declining to create in a non-active scope is
      the right behaviour for what the interpreter uses it for; it is only wrong as a *host*
      API, which is why the answer was a new entry point rather than a change to that one.
      `tests/hostvars.c` asserts the old behaviour too, so a later "tidy-up" has to argue with a
      test.

---


### Found while auditing this section against the C port

Writing the `FOR`/`NEXT` coverage that item 4 was gated on turned these up. The first two are
the reference's semantics reproduced faithfully; the third is the port's own.

18. ~~**`EXIT` on the first pass through a loop restarts the program.**~~ **Fixed.** `EXIT`
    jumped to `loopExitLine`, which is only ever written by `NEXT` — so an `EXIT` before any
    `NEXT` had run jumped to line 0. Each restart entered the `FOR` again and took another
    environment and another variable, so what a reader saw was "Maximum runtime variables
    reached" reported against the `FOR`, on a four-line program.

    It now skips forward to the `NEXT` using the same `waitingForCommand` machinery a
    zero-iteration body uses, and the `NEXT` pops on an `exiting` flag. Neither verb has to know
    where the loop ends. `tests/for_next.c`.

19. **A `FOR` body runs once with the overshot counter**, `FOR I = 1 TO 1` does not run its
    body at all, and the counter does not survive the loop. The first two compensate for a step
    of 1, which is why neither was noticed. Pinned by `tests/for_semantics.c` in
    `AKBASIC_KNOWN_FAILING_TESTS`; fixing it means deciding what to do with
    `forloopwaitingforcommand.bas`, which depends on the current behaviour. **Issue #5.**

### Found while fixing the DEF recursion hang

27. ~~**A call leaked one variable slot per parameter.**~~ **Fixed.**
    `akbasic_runtime_prev_environment()` released the scope but not the variables the scope
    created, so a subroutine with a local of its own exhausted the 128-slot pool after
    about 128 calls -- `Maximum runtime variables reached`, on a four-line program that
    does nothing unusual.

    **It was there for `GOSUB` all along**, measured on a build stashed back rather than
    assumed: `FOR I# = 1 TO 300 : GOSUB 100 : NEXT` where the subroutine assigns a local
    failed before this work and passes after it. Giving each `DEF` call its own scope
    (item 26) simply made it reachable a second way, which is how it was found.

    The release is safe because a scope's own table holds *only* what it created --
    `akbasic_environment_get()` walks up to find an outer variable and returns it without
    caching a reference. That is worth knowing before anyone adds caching there, and the
    loop says so. It is the variable *slot* that comes back, not its storage: the value
    pool never frees, so a pointer into a record `DIM`med in a scope stays sound after the
    scope is gone, which is a documented property of structures rather than an accident
    this could have taken away.

28. ~~**A function could not take a structure.**~~ **Done.**
    `DEF AREA(S@ AS RECT)` and `DEF POKEIT(P@ AS PTR TO RECT)` now work, and a bare
    `DEF F(S@)` is refused: `@` says "a structure" without saying which, so it does not
    state a contract the way `S$` does. Accepting it would mean checking fields at the
    call rather than at the declaration -- duck typing, and the hole naming the type
    closes. **The cost is that there are no generic functions**, one `AREA` cannot serve
    two types, and that is a real loss recorded rather than glossed.

    Passing is by value because a parameter is bound by assignment and assignment copies;
    a pointer parameter copies its reference. Neither is a special rule. A structure
    parameter needs its storage prepared before the copy, which is the one thing a
    primitive parameter does not -- a structure variable is a run of slots and there is
    nothing to copy into until the run exists.

    **`akbasic_value_is_truthy()` learned that a pointer is true when it points at
    something**, which had to come with it: without it there is no way to test for the end
    of a list at all, and comparing a pointer to 0 reads a numeric field it does not carry.
    A *structure* is deliberately given no truth value -- it always exists, so the question
    has no answer worth guessing at.

25. **A statement containing a failed multi-line `DEF` call still completes, and prints a junk
    value.** `akbasic_runtime_process_line_run()` swallows a script's error by design, so the
    loop inside `akbasic_runtime_user_function()` sees the run end normally. It is a decision
    about what a failed call *evaluates to*. **Issue #7.**

### Found while building structures

23. ~~**A host could not run one script twice.**~~ **Fixed.**
    `akbasic_runtime_start()` never rewound: `RUN` sets `nextline = 0` and clears
    `stopped`, and `start()` set neither. That cost nothing while a host started a script
    once, because `nextline` is already 0 the first time — and cost everything to a host
    running one script per enemy, where the second `start()` began past the end and
    silently did nothing at all. `start` means start now. Found by writing
    `examples/hoststruct.c`, which is exactly that shape.

24. ~~**The prescan wiped host-registered types.**~~ **Fixed.** It runs again on every
    `RUN` and used to reinitialise the whole table, so the first `RUN` unregistered
    everything the host had registered before loading — with no way for the host to know
    it had to register them again. It now drops what the *script* declared and keeps what
    the *host* registered, which works because host types are always a prefix and so every
    index a field already recorded keeps pointing at the same type.
    `tests/hoststruct.c` pins it.

26. ~~**A `DEF` call was not re-entrant, which cost a wrong answer and a hang.**~~
    **Fixed.** The function's environment was owned by the funcdef and re-initialised on
    every call, so a second call trampled the first:

    - **Two calls in one expression aliased one slot.** `DBL(10) + DBL(1)` was 4 rather
      than 22 -- both operands became the last call's answer, because the result was a
      pointer into the funcdef's own environment. Two *different* functions were fine,
      which is most of why it was invisible. Same severity class as §6 item 12, and the
      same shape: a silent wrong answer from a two-line program.
    - **Recursion never came back.** The recursive call re-initialised the environment the
      outer call was still using, so the loop waiting for control could not see it return.
      No error, no bound, no diagnostic -- the one place in this interpreter that looped
      forever instead of raising.

    A call takes an environment from the pool now, exactly as `GOSUB` does, and the result
    is copied into a caller-scope scratch before that environment goes back. `RETURN`
    parks its result on the *parent* rather than on the environment about to be released,
    so nothing reads a freed slot. Recursion depth answers to `AKBASIC_MAX_ENVIRONMENTS`
    like every other nesting, so too deep is `Environment pool exhausted` -- a diagnosis
    where there was none.

    `akbasic_FunctionDef.environment` is gone with it, as dead state.
    `tests/user_functions.c` and `tests/language/functions/recursion.bas`.

### Found while auditing `akbasic_value_clone()` for the structures work

22. ~~**The numeric operators' `else` was a catch-all, not a float branch.**~~ **Fixed.**
    `math_minus`, `math_multiply` and `math_divide` were `if ( INTEGER ) … else <treat as
    float>`, and that `else` read `floatval` from whatever it was handed. A truth value
    keeps its payload in `boolvalue` and leaves `floatval` zero, so a truth value on the
    *left* computed into a field nothing reads and kept its `BOOLEAN` type:

    ```
    (A# == 1) - 1      printed  true      should be -2
    (A# == 1) * 3      printed  true      should be -3
    (A# == 1) + 1      correctly refused
    ```

    Wrong in value and in type, and silent. `math_plus` escaped only because it enumerates
    its cases and ends in an error; the other three now share one `require_numeric()` guard
    so a type added later is refused by all of them at once instead of quietly taking the
    float branch in each.

    **The two operands have different rules, deliberately.** The left one picks the branch
    and must be a number. The right one is read through `rval_as_int()`, which handles a
    truth value on purpose — `5 - (A# == 1)` is 6, and that is the same property that lets
    `AND` and `OR` double as logical operators. Refusing it on the right would have broken
    every condition in the language, and `tests/language/numeric/truth_value_arithmetic.bas`
    pins both halves.

    **Found by asking what a structure operand would do**, since `AKBASIC_TYPE_STRUCT` would
    have taken exactly this path and read a `floatval` it does not carry. The structures
    work did not create this defect; it made an already-reachable one worth chasing. Same
    shape as item 5 and fixed the same way, with the assertions against the value API where
    the bad value can actually be constructed.

    Two stale comments went with it: `src/value.c`'s file header and a duplicate block above
    `rval_as_int()` both cited "TODO.md section 12", which does not exist — the defect list
    is §6 — and the duplicate described the summing behaviour item 5 had already removed.

### Found while writing the error-code appendix

21. **A dependency's status code reaches `ER#` where the interpreter has one of its own.**
    `VAL("XYZ")` reports `AKERR_VALUE` rather than `AKBASIC_ERR_VALUE`, and the errno-derived
    number is not portable across machines where 517 would have been. A decision about a
    boundary rather than a patch -- the disk verbs are the counter-case, since `ENOENT` out of
    `DOPEN` is more useful than a flat 519. **Issue #13.**

### Found while making line numbers optional

26. **`akbasic_environment_set_label()` files into the active scope, not the root**, so a
    prescanned label and a re-filed one behave differently. One condition, plus a test. Wants
    checking against deviation 32 first. **Issue #9.**

27. **Three of the four prescans report against the wrong line**, and work around it by
    putting `Line %d:` in the message text -- so the message names the number twice and the
    prefix is wrong. `akbasic_runtime_check_targets()` shows the fix. **Issue #10.**

28. **Two lines carrying the same written line number still silently keep the last.** The one
    collision that can appear in an existing program, so it carries corpus risk the other three
    did not. **Issue #11.**

29. **`akbasic_renumber()` keeps two 9999-entry `static` locals**, so `RENUMBER` and the target
    prescan are not reentrant across two runtimes in one process. Hanging them off the runtime
    costs ~2.5MB per interpreter for something one verb uses; both answers are defensible.
    **Issue #12.**

### Mutation-testing gaps in the files the game work touched

Recorded the way §8's CI note records `src/audio_tables.c`: a real test gap rather than a
reason to avoid the file.

34. **`src/runtime.c` has never had a complete mutation run** -- the one attempted was cut off
    at 551 of 997 mutants. What it covered has been read rather than filed unopened, and three
    of the four skipped-block survivors are equivalent mutants. **Issue #24.**

### Found while writing a game against the interpreter

A complete Breakout, sprites and text grid and keyboard, running for minutes at a time. Nothing
in either corpus runs for minutes, which is why the first of these had never been seen.

30. ~~**A variable created inside a scope costs pool slots that never come back, so a
    long-running program has a hard budget of 4096 name creations.**~~ **Done** — a scalar now
    lives in the variable rather than in the pool (`akbasic_Variable::inlinevalue`,
    `src/variable.c`), so a `GOSUB` local, a `FOR` counter and a `DEF` parameter all cost
    nothing at all. Twenty thousand scoped creations where four thousand used to be the whole
    run. `tests/value_pool.c` is the coverage, and it asserts the mechanism as well as the
    consequence — a later change that moved arrays inline too would pass every behavioural
    case and break the pointer guarantee. **An array created in a scope still leaks**, which is
    deliberate and is the same statement `akbasic_ValuePool` has always made: a pointer into a
    record outlives the scope that DIMmed it. The record below is what was found and why.

    **Also fixed with it:** `SWAP` exchanges whole variable records, so the `values` pointer
    that came over named the other variable's inline slot and SWAP silently did nothing. Caught
    by `tests/language/housekeeping/verbs.bas`, which is the golden corpus earning its keep.

    The original report: `akbasic_ValuePool` is a bump allocator
    on purpose (`include/akbasic/value.h:30`), and the reason given is that "nothing in BASIC
    destroys a variable". **Scope exit destroys variables.**
    `akbasic_runtime_prev_environment()` (`src/runtime.c:121`) marks a popped environment's
    variables `used = false`; `akbasic_runtime_new_variable()` (`src/runtime.c:31`) `memset`s
    the slot it hands back, clearing `values` and `valuecount`; and `akbasic_variable_init()`
    (`src/variable.c:98`) therefore takes *fresh* slots for a variable whose old ones are still
    counted against `obj->next`. Nothing decrements it, so every `GOSUB` that creates a local
    leaks the local's slots for the life of the run.

    Reduced, and it fails on both builds:

    ```basic
    X# = 0
    FOR T# = 1 TO 6000
      GOSUB SUBA
    NEXT T#
    PRINT "OK " + X#
    END

    LABEL SUBA
    LOC# = 1
    X# = X# + LOC#
    RETURN
    ```

    `? 110 : RUNTIME ERROR Array of 1 elements does not fit in the 0 remaining value slots`,
    at `LOC# = 1`, on the 4091st call -- 4096 less the handful of names the program itself
    declared. Both builds, same line, same count. Declare `LOC#` alongside `X#` and the same
    6000 calls cost nothing at all. A `FOR` counter behaves the same way: name one that does
    not already exist and every entry to the loop spends a slot.

    **What it costs a program:** a game loop that creates one name per tick is dead in half a
    minute, and the error names the line that happened to be unlucky rather than the line that
    caused it. Breakout ran twenty-five seconds before this was understood. The workaround is
    real and not onerous -- declare every name, loop counters included, at the top -- but it
    has to be *known*, and right now the only way to learn it is to hit it.

    Three ways out, in increasing order of work. Reclaim on scope exit: give the pool a mark
    and release, take the mark when an environment is pushed and rewind to it when it pops,
    which is exactly the discipline the environments themselves already have and is why they
    are a pool in the first place. Or keep the slice on the recycled variable: stop the
    `memset` in `new_variable()` from clearing `values`/`valuecount` and let
    `variable_init()`'s existing "reuse the slice when it is already big enough" branch do the
    work -- much smaller, and it fixes the common case of the same-sized scalar being recreated
    over and over. Or, at minimum, say so in the manual and in `value.h`'s comment, which
    currently states the opposite.

    Whichever is chosen, the test is a CTest program that enters and leaves a scope creating a
    variable tens of thousands of times and returns zero -- and it wants to exist even for the
    documentation-only outcome, pinned at whatever the real budget turns out to be. Item 33
    has to be settled with it: while the pool is dry the failure cannot be trapped, so a
    program cannot even report this one against itself.

31. ~~**`WINDOW` cannot be reached in the standalone AKGL build.**~~ **Done** — `tee_window()`
    (`src/sink_tee.c`