What is the highest number of pieces from one side that can be involved in a checkmate? "Involved in a checkmate" is defined below.
A non-king piece is involved in a checkmate 'if removing it from the checkmate position would change the position to one which is either not checkmate or is illegal'. The king is involved in a checkmate if it is the only piece of the checkmating side that attacks one of the other king's escape squares.
