anchor.diff:  (defclass pvector ()
     ((x :initarg :x :initform 0 :accessor x)
      (y :initarg :y :initform 0 :accessor y)))
  +
  +(defclass mover ()
  +  ((location :initarg :location :initform nil :accessor location)
  +   (velocity :initarg :velocity :initform nil :accessor velocity)
  +   (topspeed :initarg :topspeed :initform nil :accessor topspeed)))
  +
   
   (defun equal-vectors (vec-1 vec-2)
     (setf (x vec-1) (x vec-2))
  
positive.code:   (format t "> motion-101~%")
    (format t "> motion-101-acceleration~%")
    (format t "> motion-101-random-acceleration~%")
    (format t "> motion-101-acceleration-towards-mouse~%"))
  
negative.code: (defparameter *acceleration* nil)
  (defparameter *mouse* nil)
  (defparameter *center* nil)
  (defparameter *movers* nil)
  
  (defparameter *x* 0)
  (defparameter *y* 0)
  