class ['a] union_set set1 set2 =
  let _ = if set1#superset <> set2#superset then
    failwith "The two sets given to union_set must be subsets of the same set."
  else () in
  object(self)
    inherit ['a] subset
      set1#superset
      (fun x -> (set1#contains x) || (set2#contains x))
  end
;;