Translate the following inference to logic notation: Charles is sad or it is not overcast. For all x, if x_15 were sad, then x_15 is a janitor. If Charles were a janitor, then it is overcast. Therefore it is overcast if and only if Charles is sad.
P(a) or ~p. forall x: P(x) -> Q(x). Q(a) -> p. Therefore p <-> P(a).