Consider the following premises. s_1 -> u. u -> t_1. t_1 <-> t. ~t. Can we infer s_1 from them?
 From s_1 -> u, u -> t_1 we can infer s_1 -> t_1. From t_1 <-> t, ~t we can infer ~t_1. Finally, from s_1 -> t_1, ~t_1 we can infer ~s_1, which contradicts s_1. Therefore, the answer is no.