Translate the following inference to logic notation: For all x, if y_4 plays a game, then y_4 is sad. For all x, if y_4 were sad, then y_4 is playing a game. Susan is not sad. Therefore Susan is not playing a game.
forall x: P(x) -> Q(x). forall x: Q(x) -> P(x). ~Q(a). Therefore ~P(a).