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 -> r. ~r. r -> w. r -> w.
~w can be inferred via the modus tollens rule. w <-> r can be inferred via the biconditional introduction rule. w <-> r can be inferred via the biconditional introduction rule. r <-> w can be inferred via the biconditional introduction rule. r <-> w can be inferred via the biconditional introduction rule.