Give each DEF call its own environment, so recursion returns

The function's environment was owned by the funcdef and re-initialised on every
call, which made a function not re-entrant and cost two silent defects:

    DEF DBL(N#) = N# * 2
    PRINT DBL(10) + DBL(1)      was 4, should be 22

The result was a pointer into the funcdef's own environment, so the second call
overwrote the first before the operator saw it -- both operands became the last
call's answer. Two *different* functions in one expression were fine, which is
most of why it was invisible.

    DEF FACT(N#)
    IF N# <= 1 THEN RETURN 1
    RETURN N# * FACT(N# - 1)
    PRINT FACT(5)               never returned

The recursive call re-initialised the environment the outer call was still
using, so the loop waiting for control to come back could not see it. No error,
no bound, no diagnostic -- the one place in this interpreter that looped forever
rather than raising.

A call takes an environment from the pool now, exactly as GOSUB does. The result
is copied into a caller-scope scratch before that environment goes back, because
handing back a pointer into the callee is what made two calls collide and would
now be a pointer into a released slot as well. RETURN parks its result on the
*parent* rather than on the environment it is about to release, so nothing reads
a freed slot to find it.

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 goes with it, as dead state.

One thing this exposed but did not cause, measured against a stashed build and
recorded rather than fixed: a statement containing a failed multi-line DEF call
still completes and prints a junk value. It is visible more often now only
because runaway recursion reaches it where it used to hang.
tests/language/functions/recursion.bas deliberately does not pin that answer.

Chapter 16 loses its "walk a list with a loop, not a recursive DEF" caveat and
gains the one that is still true: a function cannot take a structure parameter
yet, so it reaches a record by name.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -300,6 +300,7 @@ set(AKBASIC_TESTS
   struct_types
   trap_verbs
   symtab
+  user_functions
   value_arithmetic
   value_bitwise
   value_compare

TODO.md

@@ -1573,6 +1573,34 @@ the reference's semantics reproduced faithfully; the third is the port's own.
     listings read it there. Same known-failing test as item 19; the fix is a scoping decision
     about where a loop counter is created, not an ordering one.
 
+### Found while fixing the DEF recursion hang
+
+25. **A statement containing a failed multi-line `DEF` call still completes, and prints a
+    junk value.** The call's error is reported and the run stops, but the *enclosing*
+    statement carries on with whatever `*dest` was left holding:
+
+    ```
+    10 DEF BAD(N#)
+    20 RETURN N# / 0
+    30 PRINT 99
+    40 PRINT BAD(1)
+    ```
+
+    prints `99`, the division-by-zero line, and then
+    `(UNDEFINED STRING REPRESENTATION FOR 0)`.
+
+    **Pre-existing, and measured as such**: identical before and after the re-entrancy
+    fix, on a build stashed back to compare. It is visible more often now only because
+    runaway recursion reaches it where it used to hang instead.
+
+    The cause is that `akbasic_runtime_process_line_run()` swallows a script's error by
+    design -- goal 3 -- so the loop inside `akbasic_runtime_user_function()` sees the run
+    end normally and hands back a value nobody should use. The fix is for the multi-line
+    path to notice the run did not return through `RETURN` and refuse rather than produce
+    a value; it is not done here because it is a decision about what a failed call
+    *evaluates to*, and `tests/language/functions/recursion.bas` deliberately does not
+    pin the current answer.
+
 ### Found while building structures
 
 23. ~~**A host could not run one script twice.**~~ **Fixed.**
@@ -1591,6 +1619,30 @@ the reference's semantics reproduced faithfully; the third is the port's own.
     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.**
@@ -1712,12 +1764,12 @@ requirement; exactly one case has diverged on purpose since, and
 
 | Gate | Result |
 |---|---|
-| `ctest` | 104/104 — 41 reference golden cases, 20 local ones, 39 unit tests, 3 embedding examples, and `docs_examples` |
+| `ctest` | 106/106 — 41 reference golden cases, 20 local ones, 39 unit tests, 3 embedding examples, and `docs_examples` |
-| `ctest` with `-DAKBASIC_WITH_AKGL=ON` | 104/104 on a machine with a display; 103 passed + 1 skipped headless. The same set minus the three `no_device` cases the SDL driver contradicts, plus `akgl_backends`, `akgl_frontend`, `docs_screenshots` and `akgl_typing` — the last of which is the skip, and the `akgl_build` CI job is where it skips |
+| `ctest` with `-DAKBASIC_WITH_AKGL=ON` | 106/106 on a machine with a display; 105 passed + 1 skipped headless. The same set minus the three `no_device` cases the SDL driver contradicts, plus `akgl_backends`, `akgl_frontend`, `docs_screenshots` and `akgl_typing` — the last of which is the skip, and the `akgl_build` CI job is where it skips |
 | `docs_examples` | Every fenced block in `README.md`, `MAINTENANCE.md` and `docs/` executed and byte-compared: 49 programs, 9 transcripts, 57 output comparisons, 2 excerpts, 2 shell blocks and 8 figures in the default build; 65 programs and 56 output comparisons in the AKGL one. The C-snippet count reads 0 when the harness is run by hand without `--cflags-file`; CTest passes it. `MAINTENANCE.md` documents the fence-tag convention |
 | `docs_screenshots` | 8/8 figures re-rendered and byte-identical to the checked-in PNGs. AKGL build only — rendering a picture needs the SDL half |
 | Golden corpus | 41/41 byte-exact from `tests/reference/` — **and 41/41 again through the SDL binary**, which is most of what proves the frontend changes no output |
-| ASan + UBSan | 104/104 |
+| ASan + UBSan | 106/106 |
 | Line coverage | 94.8% (6648/7013) — above the 90% gate |
 | Function coverage | 98.4% (441/448) |
 | Warnings | none under `-Wall -Wextra` |

docs/16-structures.md

@@ -186,9 +186,11 @@ Note line 130: assigning one *pointer* to another copies the reference, not the
 That is the one place assignment does not deep-copy, and it is why pointers are declared
 separately rather than being a mode a structure can be in.
 
-**Walk a list with a loop, not with a recursive `DEF`.** A recursive multi-line `DEF`
+A recursive `DEF` works too, and is bounded by the scope pool at 32 deep — the same
-does not return in this interpreter; it is a known defect recorded in `TODO.md`, and it
+bound `GOSUB` has. What a function cannot yet do is take a structure *parameter*:
-is not specific to structures.
+`DEF F(B@ AS NODE)` is not implemented, and a bare `DEF F(B@)` is refused because `@`
+alone does not say which type. Until it is, a function reaches a record by name, which
+the scoping already allows — a call can see the caller's variables.
 
 `PRINT` follows pointers, so a cycle would not come back — it stops after four levels and
 prints `(...)`.

include/akbasic/runtime.h

@@ -91,7 +91,12 @@ typedef struct
     akbasic_ASTLeaf *arglist;
     akbasic_ASTLeaf *expression;
     int64_t lineno;
-    akbasic_Environment *environment;
+    /*
+     * There is deliberately no environment here. It used to be owned by the
+     * funcdef and reset on every call, which made a function not re-entrant --
+     * two calls in one expression shared one slot, and recursion never came
+     * back. A call takes one from the pool now, exactly as GOSUB does.
+     */
     /* Deep copies of the arglist and expression need storage that outlives the line. */
     akbasic_ASTLeaf leafstorage[AKBASIC_MAX_LEAVES * 2];
     akbasic_LeafPool leafpool;

src/runtime.c

@@ -767,55 +767,85 @@ akerr_ErrorContext *akbasic_runtime_user_function(akbasic_Runtime *obj, akbasic_
     akbasic_ASTLeaf *argptr = NULL;
     akbasic_Value *argvalue = NULL;
     akbasic_Value *unused = NULL;
+    akbasic_Value *result = NULL;
+    akbasic_Value *out = NULL;
+    akbasic_Environment *callenv = NULL;
     void *fnptr = NULL;
 
     PASS(errctx, akbasic_environment_get_function(obj->environment, expr->identifier, &fnptr));
     fndef = (akbasic_FunctionDef *)fnptr;
 
     /*
-     * The function's environment is owned by the funcdef, not by the pool free
+     * **One environment per call, from the pool -- exactly as GOSUB does.**
-     * list: it is reset on every call and outlives any single one. The reference
+     *
-     * holds it by value inside BasicFunctionDef for the same reason.
+     * It used to be owned by the funcdef and reset on every call, which made a
+     * function not re-entrant and cost two silent defects:
+     *
+     *   - Calling one twice in a single expression aliased one storage slot, so
+     *     the second call overwrote the first result before the operator saw it.
+     *     `DBL(10) + DBL(1)` was 4 rather than 22, and two different functions
+     *     in one expression were fine, which is what made it so hard to see.
+     *   - Recursion did not terminate. The recursive call re-initialised the
+     *     environment the outer call was still using, so the loop below could
+     *     never see control come back. No error, no bound, no diagnostic -- the
+     *     one place in this interpreter that hung instead of raising.
+     *
+     * Taking it from the pool fixes both, and makes recursion depth answer to
+     * AKBASIC_MAX_ENVIRONMENTS like every other nesting: too deep is now
+     * "Environment pool exhausted", which is a diagnosis rather than a hang.
      */
-    if ( fndef->environment == NULL ) {
+    PASS(errctx, akbasic_runtime_new_environment(obj));
-	PASS(errctx, akbasic_runtime_new_environment(obj));
+    callenv = obj->environment;
-	fndef->environment = obj->environment;
+    obj->environment = targetenv;
-	obj->environment = targetenv;
-    }
-    PASS(errctx, akbasic_environment_init(fndef->environment, obj, obj->environment));
 
-    /* Bind arguments into the function's scope before entering it. */
+    /*
+     * Bind arguments into the call's scope, evaluating each one in the caller's.
+     * Passing is by value: assignment is what copies, so a structure argument
+     * deep-copies and a pointer argument copies its reference.
+     */
     leafptr = (expr->right != NULL ? expr->right->right : NULL);
     argptr = (fndef->arglist != NULL ? fndef->arglist->right : NULL);
     while ( leafptr != NULL && argptr != NULL ) {
-	akbasic_Environment *callerenv = obj->environment;
 	PASS(errctx, akbasic_runtime_evaluate(obj, leafptr, &argvalue));
-	obj->environment = fndef->environment;
+	obj->environment = callenv;
-	PASS(errctx, akbasic_environment_assign(fndef->environment, argptr, argvalue, &unused));
+	PASS(errctx, akbasic_environment_assign(callenv, argptr, argvalue, &unused));
-	obj->environment = callerenv;
+	obj->environment = targetenv;
 	leafptr = leafptr->next;
 	argptr = argptr->next;
     }
 
-    obj->environment = fndef->environment;
+    obj->environment = callenv;
 
     if ( fndef->expression != NULL ) {
-	PASS(errctx, akbasic_runtime_evaluate(obj, fndef->expression, dest));
+	PASS(errctx, akbasic_runtime_evaluate(obj, fndef->expression, &result));
-	obj->environment = obj->environment->parent;
+	/*
+	 * Copied into the *caller's* scratch before the call's environment goes
+	 * back to the pool. Handing back a pointer into the callee is what made
+	 * two calls in one expression collide, and it would now be a pointer
+	 * into a released slot as well.
+	 */
+	PASS(errctx, akbasic_runtime_prev_environment(obj));
+	PASS(errctx, akbasic_environment_new_value(targetenv, &out));
+	PASS(errctx, akbasic_value_clone(result, out));
+	*dest = out;
 	SUCCEED_RETURN(errctx);
     }
 
     /*
      * A multi-line subroutine. Hand control to its environment and let the
-     * caller's step loop run it until RETURN pops back out. The result is the
+     * caller's step loop run it until RETURN pops back out. RETURN parks its
-     * value RETURN parked in the child environment.
+     * result on the *parent* -- this environment -- precisely so it outlives the
+     * pop, and it is copied into a fresh scratch here for the same reason the
+     * single-expression form does.
      */
-    obj->environment->gosubReturnLine = obj->environment->lineno + 1;
+    callenv->gosubReturnLine = callenv->lineno + 1;
-    obj->environment->nextline = fndef->lineno;
+    callenv->nextline = fndef->lineno;
     while ( obj->environment != targetenv && obj->mode == AKBASIC_MODE_RUN ) {
 	PASS(errctx, akbasic_runtime_process_line_run(obj));
     }
-    *dest = &fndef->environment->returnValue;
+    PASS(errctx, akbasic_environment_new_value(targetenv, &out));
+    PASS(errctx, akbasic_value_clone(&targetenv->returnValue, out));
+    *dest = out;
     SUCCEED_RETURN(errctx);
 }
 

src/runtime_commands.c

@@ -171,7 +171,16 @@ akerr_ErrorContext *akbasic_cmd_return(akbasic_Runtime *obj, akbasic_ASTLeaf *ex
     FAIL_ZERO_RETURN(errctx, (obj->environment->parent != NULL), AKBASIC_ERR_ENVIRONMENT,
 		     "RETURN from an orphaned environment");
     obj->environment->parent->nextline = obj->environment->gosubReturnLine;
-    PASS(errctx, akbasic_value_clone(result, &obj->environment->returnValue));
+    /*
+     * Parked on the *parent*, not on this environment.
+     *
+     * This one is about to be popped and released, so a caller reading a result
+     * out of it would be reading a pool slot that is free again -- safe only for
+     * as long as nothing else acquires one, which is not a property worth
+     * relying on. The parent is the caller, which is exactly who wants it, and
+     * it is still there afterwards.
+     */
+    PASS(errctx, akbasic_value_clone(result, &obj->environment->parent->returnValue));
     /*
      * Leaving an interrupt handler re-arms interrupts. Compared by identity
      * rather than by a depth counter so that a GOSUB the handler itself makes

src/runtime_housekeeping.c

@@ -49,7 +49,6 @@ static akerr_ErrorContext AKERR_NOIGNORE *clear_variables(akbasic_Runtime *obj)
     }
     for ( i = 0; i < AKBASIC_MAX_FUNCTIONS; i++ ) {
 	obj->functions[i].used = false;
-	obj->functions[i].environment = NULL;
     }
     PASS(errctx, akbasic_valuepool_init(&obj->valuepool));
 

tests/language/functions/recursion.bas

@@ -0,0 +1,23 @@
+10 REM A DEF call takes its environment from the pool, one per call, exactly as
+20 REM GOSUB does. It used to be owned by the funcdef and reset on every call,
+30 REM which cost two silent defects: two calls in one expression shared a slot,
+40 REM and recursion never came back at all.
+50 DEF FACT(N#)
+60 IF N# <= 1 THEN RETURN 1
+70 RETURN N# * FACT(N# - 1)
+80 PRINT FACT(5)
+90 REM Each frame keeps its own argument, which is what makes the unwind work.
+100 DEF SUMTO(N#)
+110 IF N# <= 0 THEN RETURN 0
+120 RETURN N# + SUMTO(N# - 1)
+130 PRINT SUMTO(4)
+140 REM And two calls to one function in one expression no longer collide.
+150 DEF DBL(N#) = N# * 2
+160 PRINT DBL(10) + DBL(1)
+170 PRINT DBL(1) + DBL(10) + DBL(100)
+180 REM Depth answers to the environment pool now, so runaway recursion reports
+190 REM "Environment pool exhausted" rather than hanging. It is not exercised here
+200 REM because the statement containing a failed call still prints a junk value
+210 REM afterwards -- a separate, pre-existing defect recorded in TODO.md, and one
+220 REM this golden file would pin if it went in. tests/user_functions.c asserts
+230 REM the message instead.

tests/language/functions/recursion.txt

@@ -0,0 +1,4 @@
+120
+10
+22
+222

tests/user_functions.c

@@ -0,0 +1,176 @@
+/**
+ * @file user_functions.c
+ * @brief Tests that a `DEF` call is re-entrant.
+ *
+ * Both of the defects here came from one cause: the function's environment used
+ * to be owned by the funcdef and reset on every call, so a second call trampled
+ * the first. A call takes one from the pool now, exactly as `GOSUB` does.
+ *
+ * Neither failure was visible in an ordinary program. The aliasing one gave a
+ * wrong number rather than an error, and *only* when the same function appeared
+ * twice in one expression -- two different functions were fine, which is what
+ * made it hard to see at all. The recursion one produced no output whatsoever.
+ * So both are pinned here against the language rather than left to a listing
+ * somebody might not write.
+ */
+
+#include <string.h>
+
+#include <akbasic/error.h>
+#include <akbasic/runtime.h>
+
+#include "harness.h"
+#include "testutil.h"
+
+/** @brief Run a program to completion, bounded so a hang fails rather than waits. */
+static akerr_ErrorContext AKERR_NOIGNORE *run_program(const char *source)
+{
+    PREPARE_ERROR(errctx);
+
+    PASS(errctx, harness_start(NULL));
+    PASS(errctx, akbasic_runtime_load(&HARNESS_RUNTIME, source));
+    PASS(errctx, akbasic_runtime_start(&HARNESS_RUNTIME, AKBASIC_MODE_RUN));
+    PASS(errctx, akbasic_runtime_run(&HARNESS_RUNTIME, 20000));
+    SUCCEED_RETURN(errctx);
+}
+
+/**
+ * @brief Two calls to one function in one expression do not share a slot.
+ *
+ * The result used to be a pointer into the funcdef's own environment, so the
+ * second call overwrote the first before the operator saw it: `DBL(10) + DBL(1)`
+ * came out as 4 rather than 22, both operands having become the last call's
+ * answer. A silent wrong number from a two-line program.
+ */
+static void test_two_calls_in_one_expression(void)
+{
+    TEST_REQUIRE_OK(run_program("10 DEF DBL(N#) = N# * 2\n"
+				"20 DEF TPL(N#) = N# * 3\n"
+				"30 PRINT DBL(10) + DBL(1)\n"
+				"40 PRINT DBL(1) + DBL(10) + DBL(100)\n"));
+    TEST_REQUIRE_STR(HARNESS_OUTPUT, "22\n222\n");
+    harness_stop();
+
+    /*
+     * Two *different* functions were always correct, because each funcdef owned
+     * its own environment. Asserted so a future fix that reintroduces sharing
+     * cannot pass by getting this case right.
+     */
+    TEST_REQUIRE_OK(run_program("10 DEF DBL(N#) = N# * 2\n"
+				"20 DEF TPL(N#) = N# * 3\n"
+				"30 PRINT DBL(1) + TPL(10)\n"));
+    TEST_REQUIRE_STR(HARNESS_OUTPUT, "32\n");
+    harness_stop();
+}
+
+/**
+ * @brief A recursive multi-line `DEF` returns.
+ *
+ * It used to hang: the recursive call re-initialised the environment the outer
+ * call was still using, so the loop waiting for control to come back never saw
+ * it. No error, no bound, no diagnostic -- the one place in this interpreter
+ * that looped forever rather than raising.
+ */
+static void test_recursion_returns(void)
+{
+    TEST_REQUIRE_OK(run_program("10 DEF FACT(N#)\n"
+				"20 IF N# <= 1 THEN RETURN 1\n"
+				"30 RETURN N# * FACT(N# - 1)\n"
+				"40 PRINT FACT(5)\n"));
+    TEST_REQUIRE_STR(HARNESS_OUTPUT, "120\n");
+    harness_stop();
+}
+
+/**
+ * @brief Each call gets its own arguments, which is what recursion needs.
+ *
+ * Distinct from the test above: a function could return the right answer for
+ * `FACT` by luck if the argument happened to be re-read before being clobbered.
+ * This one unwinds and uses the argument *after* the inner call has returned, so
+ * a shared parameter slot gives the wrong answer rather than no answer.
+ */
+static void test_arguments_are_per_call(void)
+{
+    TEST_REQUIRE_OK(run_program("10 DEF SUMTO(N#)\n"
+				"20 IF N# <= 0 THEN RETURN 0\n"
+				"30 RETURN N# + SUMTO(N# - 1)\n"
+				"40 PRINT SUMTO(4)\n"));
+    /* 4+3+2+1 = 10, and only if each frame kept its own N#. */
+    TEST_REQUIRE_STR(HARNESS_OUTPUT, "10\n");
+    harness_stop();
+}
+
+/**
+ * @brief Runaway recursion is reported, not hung.
+ *
+ * Depth now answers to the environment pool like every other nesting, so too
+ * deep is the same diagnosis `GOSUB` already gave. That is the whole change in
+ * one sentence: a bound where there was none.
+ */
+static void test_runaway_recursion_is_diagnosed(void)
+{
+    TEST_REQUIRE_OK(run_program("10 DEF INF(N#)\n"
+				"20 RETURN INF(N# + 1)\n"
+				"30 PRINT INF(1)\n"));
+    TEST_REQUIRE(strstr(HARNESS_OUTPUT, "Environment pool exhausted") != NULL,
+		 "unbounded recursion should report the pool bound, got \"%s\"", HARNESS_OUTPUT);
+    harness_stop();
+}
+
+/** @brief A single-expression function still works, and still nests. */
+static void test_single_expression_form(void)
+{
+    TEST_REQUIRE_OK(run_program("10 DEF SQ(N#) = N# * N#\n"
+				"20 PRINT SQ(4)\n"
+				"30 PRINT SQ(SQ(2))\n"));
+    TEST_REQUIRE_STR(HARNESS_OUTPUT, "16\n16\n");
+    harness_stop();
+}
+
+/**
+ * @brief A function reaches a structure through the caller's scope.
+ *
+ * **Not through a parameter**: `DEF F(B@ AS CRATE)` is not implemented, and a
+ * bare `DEF F(B@)` is refused because `@` alone does not say *which* type. So
+ * the way a function works with a record today is by name, which the dynamic
+ * scoping already allows -- a call's environment has the caller's as its parent.
+ *
+ * Pinned rather than left implicit, because it is the only route there is and a
+ * reader would otherwise reasonably assume the parameter form works.
+ */
+static void test_structures_reached_through_scope(void)
+{
+    TEST_REQUIRE_OK(run_program("10 TYPE CRATE\n"
+				"20   W#\n"
+				"30 END TYPE\n"
+				"40 DIM A@ AS CRATE\n"
+				"50 A@.W# = 5\n"
+				"60 DEF SCALED(N#) = A@.W# * N#\n"
+				"70 PRINT SCALED(3)\n"));
+    TEST_REQUIRE_STR(HARNESS_OUTPUT, "15\n");
+    harness_stop();
+
+    /* And a structure parameter is refused rather than silently misbehaving. */
+    TEST_REQUIRE_OK(run_program("10 TYPE CRATE\n"
+				"20   W#\n"
+				"30 END TYPE\n"
+				"40 DEF WID(B@) = B@.W#\n"
+				"50 PRINT 1\n"));
+    TEST_REQUIRE(strstr(HARNESS_OUTPUT, "ERROR") != NULL,
+		 "a structure parameter should be refused for now, got \"%s\"", HARNESS_OUTPUT);
+    harness_stop();
+}
+
+int main(void)
+{
+    TEST_REQUIRE_OK(akbasic_error_register());
+
+    test_two_calls_in_one_expression();
+    test_recursion_returns();
+    test_arguments_are_per_call();
+    test_runaway_recursion_is_diagnosed();
+    test_single_expression_form();
+    test_structures_reached_through_scope();
+
+    return akbasic_test_failures;
+}