Well, I really made a mess with the last commit; this one sorts it out.

src/c/payloads/stack.c

@@ -0,0 +1,66 @@
+/**
+ *  payloads/stack.h
+ *
+ *  a Lisp stack frame.
+ *
+ *  Sits in a pso4.
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#include <stdarg.h>
+
+#include "memory/node.h"
+#include "memory/pointer.h"
+#include "memory/pso.h"
+#include "memory/pso2.h"
+#include "memory/pso4.h"
+#include "memory/tags.h"
+
+/**
+ * @brief Construct a stack frame with this `previous` pointer, and arguments
+ * taken from the remaining arguments to this function, which should all be
+ * struct pso_pointer.
+ *
+ * @return a pso_pointer to the stack frame.
+ */
+struct pso_pointer make_frame( struct pso_pointer previous, ...) {
+	va_list args;
+    va_start(args, previous);
+    int count = va_arg(args, int);
+
+	struct pso_pointer frame_pointer = allocate( STACKTAG, 4);
+	struct pso4* frame = (struct pso4*)pointer_to_object( frame_pointer);
+
+	frame->payload.stack_frame.previous = previous;
+
+	// I *think* the count starts with the number of args, so there are
+	// one fewer actual args. Need to test to verify this!
+	count --;
+	int cursor = 0;
+	frame->payload.stack_frame.args = count;
+
+	for ( ; cursor < count && cursor < args_in_frame; cursor++) {
+		struct pso_pointer argument = va_arg( args, struct pso_pointer);
+
+		frame->payload.stack_frame.arg[cursor] = argument;
+	}
+	if ( cursor < count) {
+		struct pso_pointer more_args = nil;
+
+		for (; cursor < count; cursor++) {
+			more_args = cons( va_arg( args, struct pso_pointer), more_args);
+		}
+
+		// should be frame->payload.stack_frame.more = reverse( more_args), but
+		// we don't have reverse yet. TODO: fix.
+		frame->payload.stack_frame.more = more_args;
+	} else {
+		for (; cursor < args_in_frame; cursor++) {
+			frame->payload.stack_frame.arg[cursor] = nil;
+		}
+	}
+
+	return frame_pointer;
+}