sicp-3.4.lisp

(define (make-account balance key)
  (let ((try 0))
    (define (withdraw amount)
      (if (>= balance amount)
          (begin (set! balance (- balance amount))
                 balance)
          "Insufficient funds"))
    (define (deposit amount)
      (set! balance (+ balance amount))
      balance)
    (define (call-the-cops w) (error "Hit the floor!"))
    (define (dispatch k m)
      (if (eq? key k)
          (begin (set! try 0)
                 (cond ((eq? m 'withdraw) withdraw)
                       ((eq? m 'deposit) deposit)
                       (else (error "Unknown request -- MAKE-ACCOUNT" 
                                    m))))
          (begin (set! try (inc try))
                 (if (> try 7) call-the-cops (lambda (s) "Incorrect Password")))))
    dispatch))