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_1 -> t. t -> t_1. t <-> s.
t_1 <-> t can be inferred via the biconditional introduction rule. t <-> t_1 can be inferred via the biconditional introduction rule. t -> s can be inferred via the biconditional elimination rule. s -> t can be inferred via the biconditional elimination rule.