Consider the following premises. forall z: S(z) <-> U(z). forall z: ~U(z). Can we infer exists z: S(z) from them?
 From forall z: S(z) <-> U(z) we can infer forall z: S(z) -> U(z). Finally, from forall z: S(z) -> U(z), forall z: ~U(z) we can infer forall z: ~S(z), which contradicts exists z: S(z). Therefore, the answer is no.