%% Note the execution order of the clauses.
%% We should not call hexrewrite with two
%% uninstantiated arguments!

%% polyhexnhexes(X,N) :-
%%   M is N-1,
%%   nzeros(L,M),
%%   it2hexrewrite(X,[1,1,1,1,1,1],L).
%% 
%% it2hexrewrite([1,1,1,1,1,1],[1,1,1,1,1,1],[]) :-
%%   !.
%% 
%% it2hexrewrite(X,Y,[0|L]) :-
%%   it2hexrewrite(Z,Y,L),
%%   hexrewrite(X,Z).


polyhexnhexes(X,N) :-
  it2hexrewrite(X,[1,1,1,1,1,1],N).

%% By the size (number of hexes).
%% it2hexrewrite(X,Y,N) matches when X can be rewritten
%% to Y with N-1 rewrites:
it2hexrewrite([1,1,1,1,1,1],[1,1,1,1,1,1],1) :-
  !.

it2hexrewrite(X,Y,N) :-
  M is N-1,
  it2hexrewrite(Z,Y,M),
  hexrewrite(X,Z).