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: Barbara is not a nurse. Robert is not an office clerk. For all x_11, x_11 is a janitor if and only if x_11 is a nurse. If Charles were rich, then Robert is an office clerk.
Charles is not rich can be inferred via the modus tollens rule. Barbara is not a nurse and Robert is not an office clerk can be inferred via the conjunction rule. Robert is not an office clerk and Barbara is not a nurse can be inferred via the conjunction rule. For all x_11, if x_11 were a janitor, then x_11 is a nurse can be inferred via the universal biconditional elimination rule. For all x_11, if x_11 were a nurse, then x_11 is a janitor can be inferred via the universal biconditional elimination rule.