Consider the following premises. u_3 <-> R_12(a_12). ~s. S_12(a_12) -> s. forall x_12: R_12(x_12) <-> S_12(x_12). Can we infer ~u_3 from them? If possible, name the inference rules being used at each step.
 From forall x_12: R_12(x_12) <-> S_12(x_12) we can infer R_12(a_12) <-> S_12(a_12) via universal instantiation. From S_12(a_12) -> s, ~s we can infer ~S_12(a_12) via modus tollens. From R_12(a_12) <-> S_12(a_12), ~S_12(a_12) we can infer ~R_12(a_12) via biconditional elimination. Finally, from u_3 <-> R_12(a_12), ~R_12(a_12) we can infer ~u_3 via biconditional elimination. Therefore, the answer is yes.