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