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 y, y is a janitor. For all y, if y were a taxi driver, then y is not a janitor. There is at least one y for which y is poor if and only if y is a taxi driver.
For all y, y is not a taxi driver can be inferred via the universal modus tollens rule. There is at least one y for which if y were poor, then y is a taxi driver can be inferred via the existential biconditional elimination rule. There is at least one y for which if y were a taxi driver, then y is poor can be inferred via the existential biconditional elimination rule.