1. New a Game 2. Add a New set to pool 3. End turn 4. Add a invalid tile to that previous valid set in the pool 5. End turn Expected: - invalid pool - should not save the pool, do backUp() to resolve the previous pool - add back the played invalid tile to player's rack Actual: - invalid pool - do backUp(), but the backUp pool clone the linked tileSet object, when player add invalid tile to that tileSet object, backUp pool also affected - add back the played invalid tile to player's rack