; Eli's code to modify mzscheme-4's immutable pairs.

  ;; to avoid a malloc on every call, reuse a single pointer, but make
  ;; it thread-local to avoid races
  (define ptr (make-thread-cell #f))
  (define (get-ptr)
    (or (thread-cell-ref ptr)
        (let ([p (malloc _scheme 1)]) (thread-cell-set! ptr p) p)))

  ;; set a pointer to the cons cell, then dereference it as a pointer,
  ;; and bang the new value in the given offset
  (define (set-ca/dr! offset who p x)
    (if (pair? p)
      (let ([p* (get-ptr)])
        (ptr-set! p* _scheme p)
        (ptr-set! (ptr-ref p* _pointer 0) _scheme offset x))
      (raise-type-error who "pair" p)))

  (define (n-set-car! p x) (set-ca/dr! 1 'set-car! p x))
  (define (n-set-cdr! p x) (set-ca/dr! 2 'set-cdr! p x))