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_2 <-> s. r_2 -> u. s or ~t.
t -> s can be inferred via the rewriting rule. r_2 -> s can be inferred via the biconditional elimination rule. s -> r_2 can be inferred via the biconditional elimination rule.