#lang racket
;; appending two lists using list surgery

(define (append! xs ys)
  (cond [(null? xs) ys]
      [else (set-mcdr! (lastcell xs) ys) xs]))

(define (lastcell xs)
  (if (null? (mcdr xs)) xs (lastcell (mcdr xs))))

; utility function to make a mutable list (like the existing 'list' procedure)
; this takes a variable number of arguments (including 0)
(define (mlist . args)
  (if (null? args) 
      null 
      (mcons (car args) (apply mlist (cdr args)))))

(define j (mlist 1 2 3))

(define k (mlist 10 11 12 13 14 15))

; now try (append! j k)
; look at the result and also j and k