2 player
- Game is empty
- First players play
- Game is not finished
- Play: board is not more empty
- Play: piece is visible
- Play: not outside the board
- Play: not if not occupied
- Play: not the proper player turn (update to not ask the player)
v1 Init (no src, assert true) v2 new game (game.empty?) v3 play (assert pieces number) v4 get v5 turns v6 valid v7 winner ...