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: q_6 -> q. s_1 <-> u_4. ~q. u_4 -> q_6.
~q_6 can be inferred via the modus tollens rule. u_4 -> q can be inferred via the transitivity rule. s_1 -> u_4 can be inferred via the biconditional elimination rule. u_4 -> s_1 can be inferred via the biconditional elimination rule.