Transmuting kings = a king threatend by an opposing unit (i.e.,
a king in check) can only move in the same way as the threatening
unit. While not in check the king moves as normal kings.
(For example, a king currently threatened by a knight moves
only as a knight when escaping check, not as a king.)
Nightrider (N) = a piece that moves on "knight-lines", i.e., it kan make
1, 2 or 3 successive knihgt moves in the same direction.
Rose (Ro) = a piece that moves in "knight-circles". A rose on c3 on an empty board can for example reach c3-b5-c7-e8-g7-h5-g3-e2 in one move. To return to its start square (c3) is not considered to be a move.