Adku puzzle solver in Erlang

Adku puzzle.erl

-module(adku).
-export([adku/1]).

adku({ Width, Height }) -> adku({ Width - 1, Height - 1 }, [ ], { 0, 0 }).

adku({ Max_X, Max_Y } = Borders, Queue, Pos) ->
  InList = lists:member(Pos, Queue),
  if InList -> 0;  
     Pos =:= { Max_X, Max_Y } -> 1;     
     true ->
       NQueue = Queue ++ [ Pos ],
           down(Borders, NQueue, Pos)
         + up(Borders, NQueue, Pos)
         + right(Borders, NQueue, Pos)
         + left(Borders, NQueue, Pos)
  end.

down({ _, Max_Y }, _, { _, Max_Y }) -> 0;
down(Borders, Queue, { X, Y }) ->
  adku(Borders, Queue, { X, Y + 1 }).
  
up(_, _, { _, 0 }) -> 0;
up(Borders, Queue, { X, Y }) ->
  adku(Borders, Queue, { X, Y - 1 }).
  
right({ Max_X, _ }, _, { Max_X, _ }) -> 0;
right(Borders, Queue, { X, Y }) ->
  adku(Borders, Queue, { X + 1, Y }).
  
left(_, _, { 0, _ }) -> 0;
left(Borders, Queue, { X, Y }) ->
  adku(Borders, Queue, { X - 1, Y }).