Tracing the Robot Mouse ...

                    14> (MOUSE 3 1 ((2 1) (1 1) (1 2) (1 3)
                    <14 (MOUSE NIL)
                    14> (MOUSE 2 2 ((2 1) (1 1) (1 2) (1 3)
                    <14 (MOUSE NIL)
                    <13 (MOUSE NIL) ...
              <7 (MOUSE NIL)                  ; fail back to (4 4)
              7> (MOUSE 5 4 ((4 4) (4 5) (4 6) (4 7)        ; east
                8> (MOUSE 6 4 ((5 4) (4 4) (4 5) (4 6)
                  9> (MOUSE 6 5 ((6 4) (5 4) (4 4) (4 5)    ; south
                    10> (MOUSE 6 6 ((6 5) (6 4) (5 4)
                    11> (MOUSE 6 7 ((6 6) (6 5) (6 4)
                    12> (MOUSE 6 8 ((6 7) (6 6) (6 5)       ; south
                    13> (MOUSE 7 8 ((6 8) (6 7) (6 6)       ; east
                    14> (MOUSE 8 8 ((7 8) (6 8) (6 7)       ; east
                    15> (MOUSE 8 7 ((8 8) (7 8) (6 8)       ; north
                    16> (MOUSE 8 6 ((8 7) (8 8) (7 8)       ; north
                    <16 (MOUSE (CHEESE))        ; found the cheese!
                    <15 (MOUSE (N CHEESE))      ; last move was N
                    <14 (MOUSE (N N CHEESE))    ; push on operators
                    <13 (MOUSE (E N N CHEESE))    ; as we backtrack
                    <12 (MOUSE (E E N N CHEESE))
                    <11 (MOUSE (S E E N N CHEESE))
                    <10 (MOUSE (S S E E N N CHEESE))
                  <9 (MOUSE (S S S E E N N CHEESE))
                <8 (MOUSE (S S S S E E N N CHEESE))
              <7 (MOUSE (E S S S S E E N N CHEESE))
            <6 (MOUSE (E E S S S S E E N N CHEESE))
          <5 (MOUSE (N E E S S S S E E N N CHEESE))
        <4 (MOUSE (N N E E S S S S E E N N CHEESE))
      <3 (MOUSE (N N N E E S S S S E E N N CHEESE))
    <2 (MOUSE (N N N N E E S S S S E E N N CHEESE))
  <1 (MOUSE (N N N N N E E S S S S E E N N CHEESE))
(N N N N N E E S S S S E E N N CHEESE)