Consider the following premises. p_4 <-> s. p_4 <-> s_1. ~s. Can we infer ~s_1 from them? If possible, name the inference rules being used at each step.
 From p_4 <-> s, ~s we can infer ~p_4 via biconditional elimination. Finally, from p_4 <-> s_1, ~p_4 we can infer ~s_1 via biconditional elimination. Therefore, the answer is yes.