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_21: R_21(x_21) <-> S_21(x_21). ~R_21(a_21). S_21(a_21) <-> u.
S_21(a_21) -> u can be inferred via the biconditional elimination rule. u -> S_21(a_21) can be inferred via the biconditional elimination rule. forall x_21: R_21(x_21) -> S_21(x_21) can be inferred via the universal biconditional elimination rule. forall x_21: S_21(x_21) -> R_21(x_21) can be inferred via the universal biconditional elimination rule.