def _on_board(self, position):
		(x, y) = position
		return x >= 0 and y >= 0 and x < self.size and y < self.size
simply return True iff position is within the bounds of [0, self.size)
dataset/ETHPy150Open Rochester-NRT/RocAlphaGo/AlphaGo/go.py/GameState._on_board