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: forall x_20: R_20(x_20) -> S_20(x_20). t_2 <-> S_20(a_4). t_2 -> u. u -> R_20(a_4).
t_2 -> R_20(a_4) can be inferred via the transitivity rule. t_2 -> S_20(a_4) can be inferred via the biconditional elimination rule. S_20(a_4) -> t_2 can be inferred via the biconditional elimination rule.