Consider the following premises. u_2 -> p_4. ~t_1. r <-> p_4. r -> t_1. p_4 -> u_2. Can we infer ~u_2 from them? If possible, name the inference rules being used at each step.
 From p_4 -> u_2, u_2 -> p_4 we can infer p_4 <-> u_2 via biconditional introduction. From r -> t_1, ~t_1 we can infer ~r via modus tollens. From r <-> p_4, ~r we can infer ~p_4 via biconditional elimination. Finally, from p_4 <-> u_2, ~p_4 we can infer ~u_2 via biconditional elimination. Therefore, the answer is yes.