anchor.diff:  
   ;;;; Player
   
  +(defconstant player-movement-steps 3)
  +
   (defclass player ()
     ((pos :accessor pos)
      (row :initarg :row :accessor row)
      (col :initarg :col :accessor col)
      (grid :initarg :grid :accessor grid)
  -   (halo :initform (make-instance 'halo) :accessor halo)))
  +   (halo :initform (make-instance 'halo) :accessor halo)
  +   (movement-actions :initform '() :accessor movement-actions)))
   
   (defmethod initialize-instance :after ((player player) &rest initargs)
     (declare (ignore initargs))
  
positive.code:         (draw-circle (- 5 i) 30 t))
        (gl:color 0.0 0.0 1.0 (halo-value (halo player)))
        (draw-circle 5))))
  
  
  
negative.code:     (setf (pos player) (cell-center-position row col))))
  
  (defmethod update ((player player))
    (when (null (movement-actions player))
      (maybe-queue-movement-actions player))
    (when-let (action (pop (movement-actions player)))
      (funcall action))
    (halo-update (halo player)))
  
  (defmethod render ((player player))
  