list-merge!

(define (list-merge! less? lst1 lst2)
  (cond
    ((null? lst1) lst2)
    ((null? lst2) lst1)
    (else
     (let ((less12? (less? (car lst1) (car lst2))))
       (let ((lst1 (if less12? lst1 lst2))
             (lst2 (if less12? lst2 lst1)))
         (let ((result lst1))
           (let loop ((lst1 (cdr lst1))
                      (lst2 lst2)
                      (last result))
             (cond
               ((null? lst1)
                (set-cdr! last lst2)
                result)
               ((null? lst2)
                (set-cdr! last lst1)
                result)
               ((less? (car lst1) (car lst2))
                (set-cdr! last lst1)
                (loop (cdr lst1) lst2 (cdr last)))
               (else
                (set-cdr! last lst2)
                (loop lst1 (cdr lst2) (cdr last)))))))))))