Consider the following premises. p_11 -> s_2. p_11 <-> s_2. u_1 <-> s_2. ~u_1. Can we infer p_11 from them? If possible, name the inference rules being used at each step.
 From p_11 <-> s_2 we can infer s_2 -> p_11 via biconditional elimination. From p_11 -> s_2, s_2 -> p_11 we can infer p_11 <-> s_2 via biconditional introduction. From u_1 <-> s_2, ~u_1 we can infer ~s_2 via biconditional elimination. Finally, from p_11 <-> s_2, ~s_2 we can infer ~p_11 via biconditional elimination, which contradicts p_11. Therefore, the answer is no.