Huge amount of work. Does not even nearly compile, but it's nearer.

src/c/payloads/stack.c

@@ -0,0 +1,41 @@
+/**
+ *  payloads/stack.c
+ *
+ *  The execution stack.
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#include "memory/node.h"
+#include "memory/pso.h"
+#include "payloads/stack.h"
+
+/** 
+ * @brief The maximum depth of stack before we throw an exception.
+ *
+ * `0` is interpeted as `unlimited`.
+ */
+uint32_t stack_limit = 0;
+
+/**
+ * Fetch a pointer to the value of the local variable at this index.
+ */
+struct pso_pointer fetch_arg( struct pso4 *frame, unsigned int index ) {
+    struct pso_pointer result = nil;
+
+    // TODO check that the frame is indeed a frame!
+    if ( index < args_in_frame ) {
+        result = frame->payload.stack_frame.arg[index];
+    } else {
+        struct pso_pointer p = frame->payload.stack_frame.more;
+
+        for ( int i = args_in_frame; i < index; i++ ) {
+            p = pointer_to_object( p)->payload.cons.cdr;
+        }
+
+        result = pointer_to_object( p)->payload.cons.car;
+    }
+
+    return result;
+}