PhantomChess: Any unit except a king may move either normally (from its current square) or as though from its Circe rebirth square (game-array squares, of the same colour in the case of pieces, on the file of capture in the case of pawns, and on the promotion square of the file of capture in the case of fairy pieces) if the latter is vacant.

Neutral piece: Belongs to whichever side chooses to use it. It can therefore be moved or captured by White or Black, and in Circe it is reborn according to capture. A King may not be moved onto a square controlled by a neutral piece, because of self-check.