[cgsrsbeu] Pawn promotion in bughouse

A few distinct ideas of modifying pawn promotion in bughouse or crazyhouse chess.  The motivation is, because pawns can easily be captured then dropped on the 7th rank, there end up being too many queens, or the threat thereof, ruining the subtlety of the game.

A pawn can promote only to a Man, i.e., a fairy chess piece that moves like a nonroyal king.  Inspired by shogi, which also does drops, but pawns can promote only to Gold Generals which are far less powerful than chess queens.

A pawn that started on the second or third rank (either from the initial position or the being dropped there) may promote to any piece, but pawns dropped further up can only promote to rook, bishop, or knight (or Man).  This makes the promotion decision nontrivial depending on exactly what kind of piece movement you need at the moment.  We need to mark which pawns are queenable versus not.