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: u -> ~r. u <-> s. q. r.
~u can be inferred via the modus tollens rule. q and r can be inferred via the conjunction rule. r and q can be inferred via the conjunction rule. u -> s can be inferred via the biconditional elimination rule. s -> u can be inferred via the biconditional elimination rule.