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: David is taking a plane if and only if David is a janitor. For all z, z is a janitor if and only if z will take a plane.
If David takes a plane, then David is a janitor can be inferred via the biconditional elimination rule. If David were a janitor, then David is taking a plane can be inferred via the biconditional elimination rule. For all z, if z were a janitor, then z is taking a plane can be inferred via the universal biconditional elimination rule. For all z, if z takes a plane, then z is a janitor can be inferred via the universal biconditional elimination rule.