+std::ostream& operator<<(std::ostream& os, const Board& board);
+
+struct Step {
+ Board board;
+ std::shared_ptr<Step> prev;
+ std::vector<Step*> successors(std::shared_ptr<Step> shared_this);
+ static signed char adjacent[BOARD_SIZE][5];
+};
+std::ostream& operator<<(std::ostream& os, const Step& step);