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_1(x_3) -> U(x_3). forall x_3: U(x_3) -> U_1(x_3). forall x_3: U_1(x_3) -> U(x_3).
forall x_3: U(x_3) <-> U_1(x_3) can be inferred via the universal biconditional introduction rule. forall x_3: U_1(x_3) <-> U(x_3) can be inferred via the universal biconditional introduction rule. exists x_3: U_1(x_3) <-> U(x_3) can be inferred via the existential biconditional introduction rule. exists x_3: U(x_3) <-> U_1(x_3) can be inferred via the existential biconditional introduction rule.