Consider the following premises. q -> u. t <-> s. u -> s. t -> q. Can we infer q <-> s from them?
 From q -> u, u -> s we can infer q -> s. From t <-> s we can infer s -> t. From s -> t, t -> q we can infer s -> q. Finally, from q -> s, s -> q we can infer q <-> s. Therefore, the answer is yes.