anchor.diff:  
   (defparameter *movers* nil)
   (defparameter *liquids* nil)
  +(defparameter *attractors* nil)
   (defparameter *wind* nil)
   (defparameter *gravity* nil)
   (defparameter *friction* nil)
  
positive.code:   (format t "> forces-2-2~%")
    (format t "> forces-2-3~%")
    (format t "> forces-2-4~%")
    (format t "> forces-2-5~%"))
  
negative.code:     (mult d drag-mag)))
  
  
  (defun attract (a m)
    (let* ((force (make-instance 'pvector
  			       :x (- (get-x (location a)) (get-x (location m)))
  			       :y (- (get-y (location a)) (get-y (location m)))))
  	 (d (mag force))
  	 (strength 0))
  
      (setf d (maths:constrain d 5.0 25.0))
      (normalize force)
      (setf strength (/ (* (gravity a) (mass a) (mass m))
  		      (* d d)))
      (mult force strength)
      force))
  
  
  
  ;;;; help
  (defun help ()
    (format t "Introduction~%")
  