add_stone(stone(A,A),FreeSides,RestStones,Chain) :- !,
    once(select(A,FreeSides,RestFreeSides)),
    chain([A,A|RestFreeSides],RestStones,Chain).
add_stone(stone(A,B),FreeSides,RestStones,Chain) :-
    (
      once(select(A,FreeSides,RestFreeSides)),
      chain([B|RestFreeSides],RestStones,Chain)
    ;
      once(select(B,FreeSides,RestFreeSides)),
      chain([A|RestFreeSides],RestStones,Chain)
    ).