@Joonas: Are you up with the rules of chess? If a white knight delivers a checkmate to the black king, but cannot itself move because it's blocking a check, to the white king, from a black bishop, is that mate still valid?
@tony Just to be clear, this means that the knight moved to protect its king and issue a checkmate simultaneously, right? Because the other way to read what you said would be if it moved to check the king but in doing so exposed its own king. That would be impossible.
But I think you mean the former, so yes, it's valid. It helps to think about it in terms of moves. If black can't protect its king in the next move, then on white's turn it takes the king and the game is over. It doesn't matter that white's king is thus exposed because the king would already have been captured.
@cmw That line of reasoning works well here, but for some reason it doesn't always work to take the game all the way up to the king being captured. That was the first way I thought about it, too.
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