@ledja: I played a lot the windows' version in which you can play against people. I'm not sure that it's an advantage since the difficulty level is relative (depends on the player), some players quit the game unexpectedly, or the most usual issue is that some people take ages before moving a pawn. This version of Reversi aims only to be more dynamic and fun for casual gamers than the classic one.
I'd like to thank all of you guys for your good comments. I've just released a new version taking them into account and I hope especially that the Beginner level will be more suitable.
Have a nice play!
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