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: exists x_3: U_4(x_3) -> U(x_3). forall x_3: ~Q_3(x_3). forall x_3: U(x_3) -> U_4(x_3). forall x_3: U(x_3) -> Q_3(x_3).
forall x_3: ~U(x_3) can be inferred via the universal modus tollens rule. exists x_3: U_4(x_3) -> Q_3(x_3) can be inferred via the existential transitivity rule. exists x_3: U_4(x_3) <-> U(x_3) can be inferred via the existential biconditional introduction rule. exists x_3: U(x_3) <-> U_4(x_3) can be inferred via the existential biconditional introduction rule.