Translate the following inference to logic notation: For all x, x_13 is a server if and only if x_13 is a musician. For all x, x_13 will write a letter if and only if x_13 is a server. For all x, x_13 is sad. For all x, if x_13 were sad, then x_13 is writing a letter. Therefore for all x, x_13 is a server.
forall x: P(x) <-> Q(x). forall x: R(x) <-> P(x). forall x: S(x). forall x: S(x) -> R(x). Therefore forall x: P(x).