; two versions, first is more efficient methinks

(define (list-insert! lst idx val)
  (if (equal? (length lst) idx)
    (set-cdr! (list-tail lst (- idx 1)) (list val))
    (let ((sublst (list-tail lst idx)))
      (set-cdr! sublst (cons (car sublst) (cdr sublst)))
      (set-car! sublst val)))
  lst)


(define (list-with-insertion lst idx val)
  (if (zero? idx)
    (cons
      val
      (list-with-insertion lst (- idx 1) val))
    (if (null? lst)
      lst
      (cons
        (car lst)
        (list-with-insertion (cdr lst) (- idx 1) val)))))