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: s -> r. u -> r. r <-> u. r -> s. ~u.
u -> s can be inferred via the transitivity rule. s <-> r can be inferred via the biconditional introduction rule. r <-> s can be inferred via the biconditional introduction rule. r -> u can be inferred via the biconditional elimination rule. u -> r can be inferred via the biconditional elimination rule. ~r can be inferred via the biconditional elimination rule.