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 -> ~w_4. s -> t. ~t. ~s -> w_4. t -> s.
~s can be inferred via the modus tollens rule. s <-> t can be inferred via the biconditional introduction rule. t <-> s can be inferred via the biconditional introduction rule.