What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? For all z_4, if z_4 were a taxi driver, then z_4 is happy. For all z_4, if z_4 were happy, then z_4 is a taxi driver.
For all z_4, z_4 is a taxi driver if and only if z_4 is happy. For all z_4, z_4 is happy if and only if z_4 is a taxi driver.