anchor.diff:  (defparameter *pheros* nil
     "pheros either emitted by croods or dropped by thrants")
   
  -(defparameter *width* 600)
  -(defparameter *height* 480)
  -(defparameter *initial-croods* 75)
  -(defparameter *initial-thrants* 20)
  +(defparameter *width* 800)
  +(defparameter *height* 600)
  +(defparameter *initial-croods* 10)
  +(defparameter *initial-thrants* 1)
   
   (defparameter *crood-draw-radius* 2)
   (defparameter *crood-color* sdl:*white*
  
positive.code: 		 (< (third close-phero) *phero-use-radius*)
  		 (< (random 1.0) *phero-use-probability*))
  	    (progn
  	      (format t "Decided to follow a phero ...")
  	      (delete (second close-phero) *pheros*)
  	      (first close-phero))
  	    (nth (random (length valid-options)) valid-options))))))
  
  (defun maybe-drop-phero (pos)
    (if (< (random 1.0) *phero-drop-probability*)
        (format t "Decided to create a phero at ~a~%" pos)
        (push (make-instance 'phero :pos pos) *pheros*)))
  
  (defun move-thrant (thrant)
  
negative.code: 	 (old-pos (thrant-pos thrant))
  	 (new-pos (choose-new-pos old-pos move-size)))
      (when (> energy-change 0)
        (log-message :debug "Thrant moved from ~a to ~a, it's energy is now ~a ~%"
  	      old-pos new-pos (- (thrant-energy thrant) energy-change))
        (push (make-line :frompos old-pos :topos new-pos :col (thrant-col thrant))
  	    *lines*)
  