Consider the following premises. p_21. t. p_21 <-> q_21. s_1 -> ~t. Can we infer s_1 from them?
 From p_21 <-> q_21 we can infer p_21 -> q_21. Finally, from s_1 -> ~t, t we can infer ~s_1, which contradicts s_1. Therefore, the answer is no.