* of wrapped collections: full and subcollections. Both have a delegate pointing to the
   * underlying collection class.
   *
   * <p>Full collections, identified by a null ancestor field, contain all multimap values for a
   * given key. Its delegate is a value in {@link AbstractMapBasedMultimap#map} whenever the
   * delegate is non-empty. The {@code refreshIfEmpty}, {@code removeIfEmpty}, and {@code addToMap}

   * of wrapped collections: full and subcollections. Both have a delegate pointing to the
   * underlying collection class.
   *
   * <p>Full collections, identified by a null ancestor field, contain all multimap values for a
   * given key. Its delegate is a value in {@link AbstractMapBasedMultimap#map} whenever the
   * delegate is non-empty. The {@code refreshIfEmpty}, {@code removeIfEmpty}, and {@code addToMap}