Consider the following premises. s_1 or ~q. s_1 -> q. Can we infer q <-> s_1 from them? If possible, name the inference rules being used at each step.
 From s_1 or ~q we can infer q -> s_1 via rewriting. Finally, from q -> s_1, s_1 -> q we can infer q <-> s_1 via biconditional introduction. Therefore, the answer is yes.