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. r_1 <-> t. s.
~t and s can be inferred via the conjunction rule. s and ~t can be inferred via the conjunction rule. r_1 -> t can be inferred via the biconditional elimination rule. t -> r_1 can be inferred via the biconditional elimination rule. ~r_1 can be inferred via the biconditional elimination rule.