The (pseudo-)legality might depend on aspects of the game state that were not reversibly updated. Like e.p. or castling rights, side-effect captures of an Ultima Withdrawer or Long Leaper.
That's a good point. Board position is not the only thing affecting the legality of a move. For castling, it has to keep track of whether the King and Rook have moved before, and this is done with flags that change value after a King or Rook moves. If you evaluated each move only once, you would need to use constants to keep track of whether the King or Rook had moved. Since constants normally keep their value, this would have to be done with resetconst. Back before I added constants to the language, though, this wasn't an option.
That's a good point. Board position is not the only thing affecting the legality of a move. For castling, it has to keep track of whether the King and Rook have moved before, and this is done with flags that change value after a King or Rook moves. If you evaluated each move only once, you would need to use constants to keep track of whether the King or Rook had moved. Since constants normally keep their value, this would have to be done with
resetconst. Back before I added constants to the language, though, this wasn't an option.