Consider the following premises. u -> q_13. s_3 <-> r_4. ~q_13. s_3 <-> u. r_4 -> s_3. Can we infer r_4 from them? If possible, name the inference rules being used at each step.
 From u -> q_13, ~q_13 we can infer ~u via modus tollens. From s_3 <-> u, ~u we can infer ~s_3 via biconditional elimination. From s_3 <-> r_4 we can infer s_3 -> r_4 via biconditional elimination. From s_3 -> r_4, r_4 -> s_3 we can infer s_3 <-> r_4 via biconditional introduction. Finally, from s_3 <-> r_4, ~s_3 we can infer ~r_4 via biconditional elimination, which contradicts r_4. Therefore, the answer is no.