message: revert back to positional superko instead of situational, since we're playing tromp-taylor rules
diff: @@ -58,7 +58,7 @@ Position::Position(BoardVisitor* bv, GroupVisitor* gv, Color to_play, int n)
  group_visitor_(gv),
  to_play_(to_play),
  n_(n),
  - stone_hash_(zobrist::ToPlayHash(to_play)) {}
  + stone_hash_(0) {}
  Position::Position(BoardVisitor* bv, GroupVisitor* gv, const Position& position)
  : Position(position) {
  @@ -72,7 +72,6 @@ void Position::PlayMove(Coord c, Color color) {
  return;
  }
  - stone_hash_ ^= zobrist::ToPlayHash(to_play_);
  if (color == Color::kEmpty) {
  color = to_play_;
  } else {
  @@ -86,7 +85,6 @@ void Position::PlayMove(Coord c, Color color) {
  n_ += 1;
  num_consecutive_passes_ = 0;
  to_play_ = OtherColor(to_play_);
  - stone_hash_ ^= zobrist::ToPlayHash(to_play_);
  previous_move_ = c;
  }
  @@ -168,9 +166,7 @@ void Position::PassMove() {
  n_ += 1;
  num_consecutive_passes_ += 1;
  ko_ = Coord::kInvalid;
  - stone_hash_ ^= zobrist::ToPlayHash(to_play_);
  to_play_ = OtherColor(to_play_);
  - stone_hash_ ^= zobrist::ToPlayHash(to_play_);
  previous_move_ = Coord::kPass;
  }
  