anchor.diff:          (lambda (boid world)                 ; alignment
             (align boid (boids world)))
           (lambda (boid world)                 ; separation
  -          (separate boid (boids world)))))
  +          (separate boid (boids world)))
  +        (lambda (boid world)                 ; wall avoidance
  +          (avoid-walls boid))))
   
   (defun com (boid all-boids)
     "Center of mass of `BOIDS'."
  
positive.code:                               :separation)
                           1.0)
          (p2dm:make-vector-2d))))
  
  
  
negative.code: 
  ;;; Conf
  
  (defparameter +max-boid-steering+ 60.0)
  (defparameter +max-boid-speed+ 60.0)
  (defparameter +min-boid-separation+ 20)
  
  (defparameter +min-boid-distance-to-game-area-boundary+ 30)
  
  (defparameter +boid-perception-range+ 50)
  
  
  