anchor.diff:  
   (defun update-world (dt)
     (update-all-boids *world* dt)
  -  (update-player (player *world*) dt)
  +  (update-entity (player *world*) *world* dt)
   
     (handle-collisions *world*)
     (remove-eaten-food *world*))
  
positive.code:    (angular-speed :initarg :speed
                    :initform p2dm:+pi+
                    :accessor player-angular-speed)))
  
  (defmethod draw-entity ((player player))
    (with-slots (position orientation color)
  
negative.code: (defun update-all-boids (world dt)
    (let ((boids (boids world)))
      (loop for boid in boids
         do (update-entity boid world dt))))
  
  (defun draw-all-boids ()
    (loop for boid in (boids *world*)
       do (draw-entity boid)))
  
  (defun spheres-collide-p (pos1 pos2 r1 r2)
    (< (p2dm:distance-between-vectors-squared pos1 pos2)
  