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: r <-> p. p_2 or p. ~p_2 or u. r or ~u.
p or u can be inferred via the resolution rule. u -> r can be inferred via the rewriting rule. r -> p can be inferred via the biconditional elimination rule. p -> r can be inferred via the biconditional elimination rule.