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:                                        (> distance +min-boid-separation+))))
                                 all-boids))
           (too-close-cnt (length too-close)))
      (p2dm:subtract-vectors (entity-position boid)
                             (if (> too-close-cnt 0)
                                 (ddv (entity-position boid)
                                      (p2dm:scaled-vector (reduce #'p2dm:add-vectors too-close :key #'entity-position)
                                                          (/ 1.0 too-close-cnt)))
                                 (p2dm:make-vector-2d)))))
  
  
  
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)
  