Here we collect short definitions for miscellaneous fairy chess and/or
units. They were not found deserving their own special entry (as e.g.
Circe chess was). In general, they are only
included because we need them for some problem in these pages.
Duel chess If possible, the piece that made
the last move must move again. I.e. if white starts with 1. Sb1-c3,
his next move must be with this knight too. Checks are orthodox (so
even if a piece isn't the current duel-piece, it can deliver check).
If the piece can't make a legal move, then a new duel-piece can be chosen.