chapter16.lisp

(defgeneric withdraw (account amount)
  (:documentation "Withdraw the specified amount from the account. Signal an
error if the current balance is less than amount."))

(defmethod withdraw ((account bank-account) amount)
  (when (< (balance account) amount)
    (error "Account overdrawn."))
  (decf (balance account) amount))


(defmethod withdraw ((account checking-account) amount)
  (let ((overdraft (- amount (balance account))))
    (when (plusp overdraft)
      (withdraw(overdraft-account) account)
      (incf (balance account) overdraft))
    (call-next-method) ; invoke withdraw wtih same arguments on 'bank-account' superclass

;;stealing
(defmethod withdraw ((account (eql *account-of-bank-president*) amount))
  (let ((overdraft (- amount (balance account))))
    (when (plusp overdraft)
      (incf (balance account (embezzle *bank* overdraft))))
    (call-next-method)))