Consider the following premises. p_11 <-> s_1. p_11 -> s_1. p. p_11 -> ~p. Can we infer ~s_1 from them?
 From p_11 <-> s_1 we can infer s_1 -> p_11. From s_1 -> p_11, p_11 -> s_1 we can infer s_1 <-> p_11. From p_11 -> ~p, p we can infer ~p_11. Finally, from s_1 <-> p_11, ~p_11 we can infer ~s_1. Therefore, the answer is yes.