ej 3 p2

(define (tecla n x) (cond
                      ((and (= (string-length n) 0) (key=? x "\b")) n)
                      ((key=? x "\b") (substring n 0 (- (string-length n) 1)))
                      (else (string-append n x)  )))
(define (pantalla n) (place-image/align (text n 20 "indigo") 0 0 "left" "top" (empty-scene 800 60)))
(big-bang ""
          [to-draw pantalla]
          [on-key tecla])