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: forall x_21: R_21(x_21) <-> S_21(x_21). ~S_21(h). u -> R_21(h). u <-> w.
u -> w can be inferred via the biconditional elimination rule. w -> u can be inferred via the biconditional elimination rule. forall x_21: R_21(x_21) -> S_21(x_21) can be inferred via the universal biconditional elimination rule. forall x_21: S_21(x_21) -> R_21(x_21) can be inferred via the universal biconditional elimination rule.