Added 'depth' counter to stack frames. The idea is two-fold:

1. You can limit runaway recursion by binding a symbol *max_stack_depth* in the environment
2. You can limit the number of backtrace frames printed.

However, neither of these have been implemented yet.

src/ops/lispops.h

@@ -190,6 +190,10 @@ struct cons_pointer lisp_cond( struct stack_frame *frame,
                                struct cons_pointer frame_pointer,
                                struct cons_pointer env );
 
+struct cons_pointer throw_exception_with_cause( struct cons_pointer location,
+                 struct cons_pointer message,
+                 struct cons_pointer cause,
+                 struct cons_pointer frame_pointer );
 /**
  * Throw an exception.
  * `throw_exception` is a misnomer, because it doesn't obey the calling