Consider the following premises. S_12(a_12) or ~U_4(a_12). forall x_12: U_4(x_12) <-> S_12(x_12). Can we infer ~(U_4(a_12) <-> S_12(a_12)) from them? If possible, name the inference rules being used at each step.
That contradicts the premises. Therefore the answer is no.