anchor.diff:  (defun static-priority (vector priority)
     (p2dm:vector-of-length vector priority))
   
  -(defun lerp-priority (vector factor &optional (min-priority +behaviour-priority-min+) (max-priority +behaviour-priority-max+))
  -  (p2dm:vector-of-length vector (alexandria:lerp factor min-priority max-priority)))
  +(defun linear-repulsion (radius distance)
  +  (float (clamp (/ (- radius distance) radius) 0 1)))
  +
  +(defun aggressive-repulsion (radius distance)
  +  ;; TODO figure it out and make it work.
  +  )
   
   (defun make-default-boid-behaviours ()
     (list (lambda (boid world)                 ; cohesion
  
positive.code:   "Behaviour to run away from `PLAYER'."
    (or
     (when player
       (let ((pos (entity-position boid))
             (player-pos (entity-position player)))
         (static-priority (ddv pos
                               (p2dm:subtract-vectors pos player-pos)
                               :avoid-player)
                          1.0)))
     (p2dm:make-vector-2d)))
  
  
negative.code: 
  (defparameter +max-boid-steering+ 60.0)
  (defparameter +max-boid-speed+ 60.0)
  (defparameter +min-boid-separation+ 30)
  
  (defparameter +min-boid-distance-to-game-area-boundary+ 30)
  
  