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 -> s. s -> q_1. ~q_1. s -> w.
~s can be inferred via the modus tollens rule. w -> q_1 can be inferred via the transitivity rule. w <-> s can be inferred via the biconditional introduction rule. s <-> w can be inferred via the biconditional introduction rule.