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: t_3 -> u. t_3 <-> r_4. t_3 or ~u.
u -> t_3 can be inferred via the rewriting rule. t_3 -> r_4 can be inferred via the biconditional elimination rule. r_4 -> t_3 can be inferred via the biconditional elimination rule.