;; IN: two lists
;; OUT: a list which is result of combining the two lists
;; BEHAVIOR: destructively combines the two lists. Preserves original pointer

(define (append! ls1 ls2)
  (if (null? ls1) ls2
      (set-cdr! (last-pair ls1) ls2)))


(define (last-pair ls)
  (if (null? (cdr ls)) ls
      (last-pair (cdr ls))))