(define (reverse! lst)
  (let (
    (pos 0)
    (tsl '())
    )
    (set! tsl (reverse lst))
    (define (help tsl)
      (if (null? (cdr tsl))
      (begin
        (set-car! (drop lst pos) (car tsl))
        (set! pos 0)
        lst
        )
      (begin
        (set-car! (drop lst pos) (car tsl))
        (set! pos (+ 1 pos))
        (help (cdr tsl))
        )
      )
      )
    (help tsl)
    )
  )