Program Definition lub (x y: t) : t :=
    match x, y return _ with
    | OK a, OK b =>
        OK (mkeqs (EqSet.union (eqs1 a) (eqs1 b))
                  (EqSet2.union (eqs2 a) (eqs2 b)) _)
    | OK _, Error _ => y
    | Error _, _ => x
    end.

Lemma ge_lub_left: forall x y, ge (lub x y) x.