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_15, if x_15 were a writer, then x_15 is a janitor. For all x_15, x_15 is not a janitor. For all x_15, x_15 is a lawyer if and only if x_15 is a janitor. There is at least one x_15 for which x_15 is a writer or x_15 is a lawyer.
For all x_15, x_15 is not a writer can be inferred via the universal modus tollens rule. For all x_15, if x_15 were a lawyer, then x_15 is a janitor can be inferred via the universal biconditional elimination rule. For all x_15, if x_15 were a janitor, then x_15 is a lawyer can be inferred via the universal biconditional elimination rule. For all x_15, x_15 is not a lawyer can be inferred via the universal biconditional elimination rule.