anchor.diff:              current-cell))))
   
   (defun e (grid-world current-cell)
  +  "Move the robot east by one cell in the grid world."
     (let ((new-y (+ (cdr current-cell) 1)))
       (list grid-world
             (if (<= new-y (- (cadr (array-dimensions grid-world)) 1))
  
positive.code:             current-cell))))
  
  (defun se (grid-world current-cell)
    (let ((new-x (+ (car current-cell) 1))
          (new-y (+ (cdr current-cell) 1)))
      (list grid-world
  
negative.code:                      (se-sensor grid-world current-cell)
                       (sw-sensor grid-world current-cell)))
  
  ;; Check if a cell in the grid world is a wall cell.
  (defun wall-cell-p (grid-world cell)
    (let ((dimensions (array-dimensions grid-world)))
      (or (= (car cell) 0)
  