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: u_2 -> r. r -> u. u -> r. u_2 or u.
u_2 -> u can be inferred via the transitivity rule. r can be inferred via the disjunction elimination rule. r <-> u can be inferred via the biconditional introduction rule. u <-> r can be inferred via the biconditional introduction rule.