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: q <-> s. w -> q. q -> w. ~s.
w <-> q can be inferred via the biconditional introduction rule. q <-> w can be inferred via the biconditional introduction rule. q -> s can be inferred via the biconditional elimination rule. s -> q can be inferred via the biconditional elimination rule. ~q can be inferred via the biconditional elimination rule.