sicp-3.3.lisp

(define (make-account balance key)
  (define (withdraw amount)
    (if (>= balance amount)
        (begin (set! balance (- balance amount))
               balance)
        "Insufficient funds"))
  (define (deposit amount)
    (set! balance (+ balance amount))
    balance)
  (define (dispatch k m)
    (if (eq? key k)
        (cond ((eq? m 'withdraw) withdraw)
              ((eq? m 'deposit) deposit)
              (else (error "Unknown request -- MAKE-ACCOUNT" 
                           m))) 
        (error "Incorrect Password")))
    dispatch)