Add references to *quadtrees* to speed up backtracking for *parallelogram polyomiones*.
Add references to *quadtrees* to speed up backtracking for *parallelogram polyomiones*.

massimo-nocentini/competitive-programming,massimo-nocentini/competitive-programming,massimo-nocentini/competitive-programming,massimo-nocentini/competitive-programming,massimo-nocentini/competitive-programming