BIT_FIRST         938 src/eval.cpp            file = BIT_FIRST(bits);
BIT_FIRST        1824 src/eval.cpp      dist = BIT_FIRST(board->pawn_file[colour][file]&BitGE[rank]);
BIT_FIRST         503 src/pawn.cpp            file = BIT_FIRST(bits);
BIT_FIRST         504 src/pawn.cpp            rank = BIT_FIRST(board->pawn_file[me][file]);