Consider the following premises. s_1 -> t. r_6 -> u_3. u_3 -> s_1. t -> r_6. Can we infer u_3 <-> s_1 from them?
 From s_1 -> t, t -> r_6 we can infer s_1 -> r_6. From s_1 -> r_6, r_6 -> u_3 we can infer s_1 -> u_3. Finally, from u_3 -> s_1, s_1 -> u_3 we can infer u_3 <-> s_1. Therefore, the answer is yes.