How not? method makeAutoMove(mark: String[PlayerMark], rnd: Int): Game requires !this->hasWinner(); method makeExplicitMove(x: Int, y:

How not? method makeAutoMove(mark: String[PlayerMark], rnd: Int): Game requires !this->hasWinner(); method makeExplicitMove(x: Int, y: Int, mark: String[PlayerMark]): Game requires !this.board->isCellOccupied(x, y); If I'm reading it right though, it looks more like it simple doesn't execute the function at all instead of throwing etc. So yet another possible route to deal with an invalid contract.