What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: r_2 <-> s_2. u_1 -> q_15. ~q_15. u_1 or s_2.
~u_1 can be inferred via the modus tollens rule. r_2 -> s_2 can be inferred via the biconditional elimination rule. s_2 -> r_2 can be inferred via the biconditional elimination rule.