Consider the following premises. u or ~s_1. u -> s. s <-> r_1. Can we infer ~(s_1 -> r_1) from them? If possible, name the inference rules being used at each step.
 From s <-> r_1 we can infer s -> r_1 via biconditional elimination. From u -> s, s -> r_1 we can infer u -> r_1 via transitivity. From u or ~s_1 we can infer s_1 -> u via rewriting. Finally, from s_1 -> u, u -> r_1 we can infer s_1 -> r_1 via transitivity, which contradicts ~(s_1 -> r_1). Therefore, the answer is no.