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 z, if z were a cashier, then z is taking a plane. For all z, z is not a cashier. For all z, if z takes a plane, then z is a cashier.
For all z, z will not take a plane can be inferred via the universal modus tollens rule. For all z, z is a cashier if and only if z will take a plane can be inferred via the universal biconditional introduction rule. For all z, z will take a plane if and only if z is a cashier can be inferred via the universal biconditional introduction rule.