Programmation déclarative - Q4B - 08/2012

(define (offset s t)
  (letrec
      ((elements-to-reach (lambda (e stream i)
                            (if (= e (head stream))
                                (list i stream)
                                (elements-to-reach e (tail stream) (+ i 1))
                                )
                            )
                          )
       )
    (let
        ((res (elements-to-reach (head t) s 0)))
      (cons-stream (car res) (offset (cadr res) (tail t)))
      )
    )
  )