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: ~w. w -> u. u -> w. u or r_4.
~u can be inferred via the modus tollens rule. w <-> u can be inferred via the biconditional introduction rule. u <-> w can be inferred via the biconditional introduction rule.