Consider the following premises. s -> p_6. ~q_1. p_6 -> s. p_6 -> q_1. Can we infer ~s from them?
 From p_6 -> s, s -> p_6 we can infer p_6 <-> s. From p_6 -> q_1, ~q_1 we can infer ~p_6. Finally, from p_6 <-> s, ~p_6 we can infer ~s. Therefore, the answer is yes.