Translate the following inference to logic notation: For all x, x_7 is a police officer if and only if x_7 is a taxi driver. If Barbara were a police officer, then Barbara is a taxi driver. Therefore Barbara is a taxi driver if and only if Barbara is a police officer.
forall x: P(x) <-> Q(x). P(a) -> Q(a). Therefore Q(a) <-> P(a).