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: For all x_4, x_4 will not take a plane. There is at least one x_4 for which x_4 is a janitor or x_4 is a lawyer. For all x_4, x_4 is a janitor if and only if x_4 will take a plane.
For all x_4, if x_4 were a janitor, then x_4 is taking a plane can be inferred via the universal biconditional elimination rule. For all x_4, if x_4 takes a plane, then x_4 is a janitor can be inferred via the universal biconditional elimination rule. For all x_4, x_4 is not a janitor can be inferred via the universal biconditional elimination rule.