(firstBlackRook <= 3 ? 'e' : 'm') +
baseFen.substring(firstBlackRook + 1, lastBlackRook) +
(lastBlackRook >= 5 ? 'm' : 'e') +
(firstBlackRook <= 3 ? 'e' : 'm') +
baseFen.substring(firstBlackRook + 1, lastBlackRook) +
(lastBlackRook >= 5 ? 'm' : 'e') +