anchor.diff:  (defparameter *refuel-counter* 0)
   (defparameter *move-counter* 0)
   
  -(defun refuel-and-nav (loc)
  +(defun* refuel-and-nav (loc)
     "refuel-and-navigate LOC 
   Navigate to location LOC.  Repeatedly choose among the N, E, S and W actions until the robot
   reaches LOC."
  
positive.code:           (with-choice navigate-choice (dir '(N E S W))
              (action navigate-move dir))))))
  
  (defun refuel-and-nav-directly (loc)
    "refuel-and-navigate LOC 
  Navigate to location LOC.  Repeatedly choose among the N, E, S and W actions until the robot
  reaches LOC."
    (until (equal (robot-loc) loc)
  
negative.code: (def-env-accessor env ws-env
      "The environemnt for a state")
  
  (defun* nav (loc)
    "nav LOC 
  Navigate to location LOC.  Repeatedly choose among the N, E, S, W, and REFUEL actions until the
  robot reaches LOC."
  