[Practice] Common Lisp: OOP method

class_method.lisp

(defclass C1 nil
  ((a :type fixnum
      :reader get-a
      :writer set-a
      :initarg :a)))

(defparameter *i1* (make-instance 'C1 :a 1))
(defparameter *i2* (make-instance 'C1 :a 2))
(defparameter *i3* (make-instance 'C1 :a 1))

(defmethod equal= ((a C1) &rest r)
  (if (not r)
      t
      (progn
        (loop for i in r do
              (unless (equal (get-a a) (get-a i))
                (return-from equal= nil)))
        t)))

(format t "~&~A~%" (get-a *i1*)) ;; => Output: 1
(format t "~&~A~%" (equal= *i1*)) ;; => Output: T
(format t "~&~A~%" (equal= *i1* *i2*)) ;; => Output: Nil
(format t "~&~A~%" (equal= *i1* *i3*)) ;; => Output: T