In the following solitaire game, you are given an m × m board. On each of its m2 positions lies either a blue stone, a red stone, or nothing at all. You play by removing stones from the board until each column contains only stones of a single color and each row contains at least one stone. You win if you achieve this objective. Winning may or may not be possible, depending upon the initial configuration. Let SOLITAIRE = {〈G〉| G is a winnable game configuration}. Prove that SOLITAIRE is NP-complete.