anchor.diff:           (unless (instruction-cache instruction)
              (setf (instruction-cache instruction)
                    (make-callable :prim args
  -                                :cached-prim (find-user-primitive vm args))))
  +                                :cached-prim (cons nil (find-user-primitive vm args)))))
            (push (instruction-cache instruction) (vm-stack vm))
            (inc-pc vm)
            ;; for the pushed callable (5 fields):
  
positive.code:          (unless (instruction-cache instruction)
             (setf (instruction-cache instruction)
                   (make-callable :prim0 args
                                  :cached-prim (find-required-primitive vm args))))
           (push (instruction-cache instruction)
                 (vm-stack vm))
           (inc-pc vm)
  
negative.code:            (type callable callable)
             (type fixnum num-args))
    (when save-return-address
      (push (make-return-address :program-counter (the fixnum (1+ (vm-program-counter vm)))
                                 :environment (vm-current-environment vm))
            (vm-stack vm)))
    (when (and (callable-num-args callable)
  