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 <-> u. u <-> t.
t -> u can be inferred via the biconditional elimination rule. u -> t can be inferred via the biconditional elimination rule. u -> t can be inferred via the biconditional elimination rule. t -> u can be inferred via the biconditional elimination rule.