Consider the following premises. p or u_4. p <-> s. ~s. Can we infer ~u_4 from them? If possible, name the inference rules being used at each step.
 From p <-> s, ~s we can infer ~p via biconditional elimination. Finally, from p or u_4, ~p we can infer u_4 via disjunctive syllogism, which contradicts ~u_4. Therefore, the answer is no.