Translate the following inference to logic notation: For all x, if x_13 were sad, then x_13 is drinking water. For all x, x_13 will drink water if and only if x_13 will take a plane. For all x, if x_13 writes a letter, then x_13 is drinking water. For all x, if x_13 drinks water, then x_13 is writing a letter. For all x, x_13 will take a plane or x_13 is sad. Therefore for all x, x_13 will write a letter.
forall x: P(x) -> Q(x). forall x: Q(x) <-> R(x). forall x: S(x) -> Q(x). forall x: Q(x) -> S(x). forall x: R(x) or P(x). Therefore forall x: S(x).