Consider the following premises. forall x_6: ~Q_6(x_6). forall x_6: U(x_6) -> Q_6(x_6). U(a_5) <-> w. Can we infer w from them? If possible, name the inference rules being used at each step.
 From forall x_6: U(x_6) -> Q_6(x_6), forall x_6: ~Q_6(x_6) we can infer forall x_6: ~U(x_6) via universal modus tollens. From forall x_6: ~U(x_6) we can infer ~U(a_5) via universal instantiation. Finally, from U(a_5) <-> w, ~U(a_5) we can infer ~w via biconditional elimination, which contradicts w. Therefore, the answer is no.