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_9: W(x_9). forall x_9: W(x_9) -> U_4(x_9). forall x_9: U_4(x_9) -> W(x_9).
forall x_9: U_4(x_9) can be inferred via the universal modus ponens rule. forall x_9: W(x_9) <-> U_4(x_9) can be inferred via the universal biconditional introduction rule. forall x_9: U_4(x_9) <-> W(x_9) can be inferred via the universal biconditional introduction rule.