ok no specializing on &optioal

(defclass standard-point ()
		((x :initform 0 :initarg :x)
			(y :initform 0 :initarg :y)))


(defgeneric distance-origin (point))

(defmethod distance-origin ((point1 standard-point))
		(when (and (slot-exists-p point1 'x) (slot-exists-p point1 'y))
				(sqrt (+ (expt (sb-mop:standard-instance-access point1 0) 2)
													(expt (sb-mop:standard-instance-access point1 1) 2)))))

(defgeneric distance (point1 point2))

(defmethod distance ((point1 null) (point2 null)))

(defmethod distance ((point1 standard-point) point2)
		(when (and (slot-exists-p point1 'x) (slot-exists-p point1 'y))
				(sqrt (+ (expt (sb-mop:standard-instance-access point1 0) 2)
													(expt (sb-mop:standard-instance-access point1 1) 2)))))

(defmethod distance (point1 (point2 standard-point))
		(when (and (slot-exists-p point2 'x) (slot-exists-p point2 'y))
				(sqrt (+ (expt (sb-mop:standard-instance-access point2 0) 2)
													(expt (sb-mop:standard-instance-access point2 1) 2)))))

(defmethod distance ((point1 null) (point2 standard-point))
		(when (and (slot-exists-p point2 'x) (slot-exists-p point2 'y))
				(sqrt (+ (expt (sb-mop:standard-instance-access point2 0) 2)
													(expt (sb-mop:standard-instance-access point2 1) 2)))))

(defmethod distance ((point1 standard-point) (point2 null))
		(when (and (slot-exists-p point1 'x) (slot-exists-p point1 'y))
				(sqrt (+ (expt (sb-mop:standard-instance-access point1 0) 2)
													(expt (sb-mop:standard-instance-access point1 1) 2)))))

(defmethod distance ((point1 standard-point) (point2 standard-point))
		(when (and (slot-exists-p point1 'x) (slot-exists-p point1 'y)
													(slot-exists-p point2 'x) (slot-exists-p point2 'y))
  (sqrt (+ (expt (- (sb-mop:standard-instance-access point2 0) (sb-mop:standard-instance-access point1 0)) 2)
											(expt (- (sb-mop:standard-instance-access point2 1) (sb-mop:standard-instance-access point1 1)) 2)))))

(defvar p1 (make-instance 'standard-point :x -3 :y 5))
(defvar p2 (make-instance 'standard-point :x 7 :y -1))

(defun test ()
		(distance p2 p1))

* (test)
11.661903
* (distance p1 nil)
5.8309517
* (distance p1 0)
5.8309517
* (distance nil p2)
7.071068
* (distance 1 p2)
7.071068
* (distance-origin p1)
5.8309517
* (distance-origin p2)
7.071068
*