anchor.diff:    (setf *debug-vectors* '()))
   
   (defun ddv (start vector)
  +  "Draw debug (velocity) vector, i.e. a line from `START' to `START' + `VECTOR'."
     (push (cons start vector) *debug-vectors*)
     vector)
   
  
positive.code:         (boids *world*)))
  
  
  (defun steering (position velocity target)
    (let* ((desired (p2dm:subtract-vectors target position))
           (steering (p2dm:subtract-vectors desired velocity)))
      (p2dm:clamped-vector steering +max-boid-steering+)))
  
  (defun update-boid (boid dt)
  
negative.code:              (p2dm:add-to-vector velocity (p2dm:scaled-vector force dt))
               (p2dm:clamp-vector velocity +max-boid-speed+)))
        (p2dm:add-to-vector position (p2dm:scaled-vector velocity dt))
        (apply-force (steering velocity (apply-behaviours boid *world*))))))
  
  (defun draw-boid (boid)
    (with-slots (position velocity color)
  