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: u_4 -> t. forall x_15: R_15(x_15) -> S_15(x_15). t <-> R_15(a_15). ~S_15(a_15).
t -> R_15(a_15) can be inferred via the biconditional elimination rule. R_15(a_15) -> t can be inferred via the biconditional elimination rule.