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_13, if x_13 were a taxi driver, then x_13 is a janitor. For all x_13, if x_13 were a janitor, then x_13 is not a carpenter. There is at least one x_13 for which x_13 is a carpenter. For all x_13, if x_13 were a janitor, then x_13 is a taxi driver.
There is at least one x_13 for which x_13 is not a janitor can be inferred via the existential modus tollens rule. For all x_13, x_13 is a taxi driver if and only if x_13 is a janitor can be inferred via the universal biconditional introduction rule. For all x_13, x_13 is a janitor if and only if x_13 is a taxi driver can be inferred via the universal biconditional introduction rule.