Bank account implementation in CLisp

gistfile1.lisp

(defclass customer ()
  ((balance :accessor customer-balance
	    :initarg :balance)
   (acct-no :accessor customer-number
	    :initarg :ac-no)
   (name    :accessor customer-name
	    :initarg :name)))

; Constructor
(defun make-customer (acct-no name)
  (make-instance 'customer :ac-no acct-no
		 	   :name  name))

(defmethod set-balance ((customer customer) balance)
  (if (< balance 0)
      'negative-amount-error
      (setf (customer-balance customer) balance)))

(defmethod get-balance ((customer customer))
  (customer-balance customer))

(defmethod withdrawable? ((customer customer) amount)
  (if (or (> amount (customer-balance customer))
	  (< amount 0))
      nil
      t))

(defmethod withdraw ((customer customer) amount)
  (if (withdrawable? customer amount)
      (set-balance customer (- (get-balance customer) amount))
      'not-enough-funds))

; We allow for negative values in this method
; since banks are usually evil like that.
(defmethod deposit ((customer customer) amount)
  (set-balance customer (+ (get-balance customer) amount)))
  
  
(setf my-customer (make-customer "James" 1))
(set-balance my-customer 200) 	; Output 200
(get-balance my-customer) 	; Output 200
(withdrawable? my-customer 300) ; Output NIL
(withdrawable? my-customer 200) ; Output T
(withdrawable? my-customer 100) ; Output T
(withdraw my-customer 100)	; Output 100 (200 - 100 lol)
(withdraw my-customer 200)	; Output NOT-ENOUGH-FUNDS
(withdraw my-customer 300)	; Output NOT-ENOUGH-FUNDS
(deposit my-customer 100)	; Output 200
(withdraw my-customer 200)	; Output 0
(withdraw my-customer 1)	; Output NOT-ENOUGH-FUNDS