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: U(x_20) -> S_20(x_20). t -> U(a_20). u -> t. u <-> S_20(a_20).
u -> U(a_20) can be inferred via the transitivity rule. u -> S_20(a_20) can be inferred via the biconditional elimination rule. S_20(a_20) -> u can be inferred via the biconditional elimination rule.