(defclass sum ()
  ((a :accessor a :initarg :a :initform 0)
   (b :accessor b :initarg :b :initform 0)
   (c :reader c)))

(defmethod compute ((self sum))
  (setf (slot-value self 'c) (+ (a self) (b self))))

(defmethod initialize-instance :after ((self sum) &key &allow-other-keys)
  (compute self))

(defmethod (setf a) :after (val (self sum))
  (declare (ignore val))
  (compute self))

(defmethod (setf b) :after (val (self sum))
  (declare (ignore val))
  (compute self))