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. u <-> p. r <-> p_12. p_12 or ~u.
u -> p_12 can be inferred via the rewriting rule. u -> p can be inferred via the biconditional elimination rule. r -> p_12 can be inferred via the biconditional elimination rule. p -> u can be inferred via the biconditional elimination rule. p_12 -> r can be inferred via the biconditional elimination rule. ~p_12 can be inferred via the biconditional elimination rule.