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_3. u -> r_3. r_3 -> u. u <-> s.
~u can be inferred via the modus tollens rule. u <-> r_3 can be inferred via the biconditional introduction rule. r_3 <-> u can be inferred via the biconditional introduction rule. u -> s can be inferred via the biconditional elimination rule. s -> u can be inferred via the biconditional elimination rule.