Consider the following premises. forall x_3: R(x_3). t -> ~u. R(a_18) -> u. Can we infer t from them? If possible, name the inference rules being used at each step.
 From forall x_3: R(x_3) we can infer R(a_18) via universal instantiation. From R(a_18) -> u, R(a_18) we can infer u via modus ponens. Finally, from t -> ~u, u we can infer ~t via modus tollens, which contradicts t. Therefore, the answer is no.