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: If Mary were a cashier, then Thomas is taking a plane. If Thomas takes a plane, then Mary is a cashier. Thomas is not taking a plane. For all x_9, x_9 will work if and only if x_9 is a cashier.
Mary is not a cashier can be inferred via the modus tollens rule. For all x_9, if x_9 works, then x_9 is a cashier can be inferred via the universal biconditional elimination rule. For all x_9, if x_9 were a cashier, then x_9 is working can be inferred via the universal biconditional elimination rule.