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: p <-> s. u. s <-> r. p -> ~u.
~p can be inferred via the modus tollens rule. p -> s can be inferred via the biconditional elimination rule. s -> r can be inferred via the biconditional elimination rule. s -> p can be inferred via the biconditional elimination rule. r -> s can be inferred via the biconditional elimination rule.