(define-entity point ()
  :constraint-variables ((a 0))
  :lisp-variables ((m 1) (x #(0 0)) (v #(0 0)) (forces '()))
  :constraints ((= (sum forces) (* m a)))
  :after (setf v (+ v (* a *time-unit*))
               x (+ x (* v *time-unit*))))

(define-entity spring ()
  :lisp-variables (p1 p2 length k)
  :constraint-variables ((:vector f1) (:vector f2))
  :before (progn (add-force p1 f1)
                 (add-force p1 f2))
  :constraints ((= (- f1 f2) (* -1 k (- (distance p1 p2) length)))))

(define-entity weight ()
  :lisp-variables ((m 1) (g -9.8) e)
  :before (progn (add-force e (* m g))))