throw new UnsupportedOperationException();
  }

  /**
   * Not supported. <b>You are attempting to create a map that may contain a non-{@code Comparable}
   * key.</b> Proper calls will resolve to the version in {@code ImmutableSortedMap}, not this dummy
   * version.
   *
   * @throws UnsupportedOperationException always
   * @deprecated <b>Pass a key of type {@code Comparable} to use {@link

  }

  /**
   * Returns an unmodifiable <b>view</b> of the union of two sets. The returned set contains all
   * elements that are contained in either backing set. Iterating over the returned set iterates
   * first over all the elements of {@code set1}, then over each element of {@code set2}, in order,
   * that is not contained in {@code set1}.
   *
   * <p>Results are undefined if {@code set1} and {@code set2} are sets based on different

   * LinkedListMultimap#create()}, {@link TreeMultimap#create()}, and {@link
   * TreeMultimap#create(Comparator, Comparator)} won't suffice.
   *
   * <p>Note: the multimap assumes complete ownership over of {@code map} and the collections
   * returned by {@code factory}. Those objects should not be manually updated and they should not
   * use soft, weak, or phantom references.
   *