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. u_1 <-> w. u_4 -> u_1. ~s.
~w can be inferred via the modus tollens rule. u_1 -> w can be inferred via the biconditional elimination rule. w -> u_1 can be inferred via the biconditional elimination rule.