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_1, if x_1 writes a letter, then x_1 is a taxi driver. For all x_1, if x_1 were a taxi driver, then x_1 is a nurse. There is at least one x_1 for which if x_1 climbs a mountain, then x_1 is a nurse. For all x_1, x_1 will climb a mountain or x_1 will write a letter.
For all x_1, if x_1 writes a letter, then x_1 is a nurse can be inferred via the universal transitivity rule.