onyx has the interesting property that, despite having a capture rule, it seems positions never repeat in a normally played game. that is, if at least one player is trying to win then it does not appear to be possible to have an onyx position where, after a number of moves have been made, some of which are captures resulting in the removal of pieces from the board, the game returns to the same position. it seems the only way a position can be repeated in onyx is if both players conspire to bring this about. however, while experience indicates that positions do not repeat, it's not obvious why this is so. in fact, it may be possible to construct an onyx position where, with correct play, the position will repeat after a number of moves. but such a position has never been discovered and it may be the case that it's impossible to create one. therefore, one interesting challenge with onyx is to construct a position that repeats with correct play or to prove that it is impossible to do so.