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 is a sailor or x_4 will take a plane. For all x_4, if x_4 were a sailor, then x_4 is rich. There is at least one x_4 for which x_4 will take a plane if and only if x_4 is rich.
There is at least one x_4 for which if x_4 takes a plane, then x_4 is rich can be inferred via the existential biconditional elimination rule. There is at least one x_4 for which if x_4 were rich, then x_4 is taking a plane can be inferred via the existential biconditional elimination rule.