A position is valid iff there is a squence of legal moves that leads to it. In go, a legal turn consists of putting a stone on nearly any vacant point (some rul

A position is valid iff there is a squence of legal moves that leads to it. In go, a legal turn consists of putting a stone on nearly any vacant point (some rule sets prohibit suicide moves, and all prohibit exactly repeating the position at the end of your previous turn) or passing (not putting a stone, which normally happens only near the end of the game), so the only illegal board configurations are those that have groups with no adjacent vacant points ("liberties"). In chess, these moves are more constrained: each piece may move only certain directions, sometimes depending on the locations of opposing pieces. http://senseis.xmp.net/?Liberty http://senseis.xmp.net/?Liberty http://senseis.xmp.net/?Suicide http://senseis.xmp.net/?Suicide