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