[thrw21]

Number of chess pieces are dynamic so you wouldn't know if white pieces are over and next piece is black king (kings don't have a type bits)

But now I think about it, instead of a separator I could simple use 4 bits to represent how many (non pawn) chess pieces there are for each player. It is at most 15 (7 initial + 8 promotion) so only 4 bits instead of 6 bit seperator shaved another 4 bits!