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: William is cooking. For all x_13, x_13 is a server if and only if x_13 is rich. Sarah is not rich.
William is cooking and Sarah is not rich can be inferred via the conjunction rule. Sarah is not rich and William is cooking can be inferred via the conjunction rule. For all x_13, if x_13 were a server, then x_13 is rich can be inferred via the universal biconditional elimination rule. For all x_13, if x_13 were rich, then x_13 is a server can be inferred via the universal biconditional elimination rule.