Azi's Simple Tic Tac Toe

This is, yes, another clone of the ancient Tic Tac Toe thing, your wits against your beloved gadget. Four levels of difficulty, from the almost-random to the invincible. This was written at the request of my little kiddy, hence the design is simple and plain as can be; No bells, no whistles, no 3D rotating baroque textured noughts and crosses (are you nutz?