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_7: ~U(x_7). exists x_7: P(x_7) -> U(x_7). forall x_7: U(x_7) -> P(x_7).
exists x_7: ~P(x_7) can be inferred via the existential modus tollens rule. exists x_7: P(x_7) <-> U(x_7) can be inferred via the existential biconditional introduction rule. exists x_7: U(x_7) <-> P(x_7) can be inferred via the existential biconditional introduction rule.