anchor.diff:                                (p2dm:scaled-vector (reduce #'p2dm:add-vectors boids :key #'entity-velocity)
                                                     (/ 1.0 boids-cnt))
                                 :alignment)
  -                         0.5)
  +                         0.4)
           (p2dm:make-vector-2d))))
   
   (defun separate (boid all-boids)
  
positive.code:                               :separation)
                           1.0)
          (p2dm:make-vector-2d))))
  
  
  
negative.code:         (lambda (boid world)                 ; alignment
            (align boid (boids world)))
          (lambda (boid world)                 ; separation
            (separate boid (boids world)))
          (lambda (boid world)                 ; wall avoidance
            (avoid-walls boid))))
  
  (defun com (boid all-boids)
    "Center of mass of `BOIDS'."
  