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: There is at least one x_12 for which if x_12 takes a plane, then x_12 is not writing a letter. For all x_12, x_12 will take a plane or x_12 is a cashier. For all x_12, x_12 will write a letter.
There is at least one x_12 for which x_12 will not take a plane can be inferred via the existential modus tollens rule.