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 or ~t_1. t_1 or ~u. u <-> t. ~t.
t_1 -> u can be inferred via the rewriting rule. u -> t_1 can be inferred via the rewriting rule. u -> t can be inferred via the biconditional elimination rule. t -> u can be inferred via the biconditional elimination rule. ~u can be inferred via the biconditional elimination rule.