Consider the following premises. p. u <-> q_12. ~u. q_12 <-> s_1. Can we infer s_1 from them? If possible, name the inference rules being used at each step.
 From u <-> q_12, ~u we can infer ~q_12 via biconditional elimination. Finally, from q_12 <-> s_1, ~q_12 we can infer ~s_1 via biconditional elimination, which contradicts s_1. Therefore, the answer is no.