(define (mutate-procedure old proc)
    (unless (##core#check (procedure? old))
      (##sys#signal-hook #:type-error 'mutate-procedure "bad argument type - not a procedure" old))
    (let* ((n (##sys#size old))
	   (words (##core#inline "C_words" n))
	   (y (##core#inline "C_copy_block" old (make-vector words))) )
      (##sys#become! (list (cons old (proc y))))
      y) )