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: r_3 <-> s. s <-> u.
r_3 -> s can be inferred via the biconditional elimination rule. s -> u can be inferred via the biconditional elimination rule. s -> r_3 can be inferred via the biconditional elimination rule. u -> s can be inferred via the biconditional elimination rule.