assert(this->piece_on(to) == EMPTY);
assert(this->piece_on(from) == pawn_of_color(us));
assert(this->piece_on(capsq) == pawn_of_color(them));
assert(this->piece_on(to) == EMPTY);
assert(this->piece_on(from) == pawn_of_color(us));
assert(this->piece_on(capsq) == pawn_of_color(them));