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_10, x_10 is not a janitor. For all x_10, if x_10 were a janitor, then x_10 is happy. If Charles were a nurse, then Joseph is happy. For all x_10, if x_10 were happy, then x_10 is a janitor.
For all x_10, x_10 is not happy can be inferred via the universal modus tollens rule. For all x_10, x_10 is a janitor if and only if x_10 is happy can be inferred via the universal biconditional introduction rule. For all x_10, x_10 is happy if and only if x_10 is a janitor can be inferred via the universal biconditional introduction rule.