code: @SuppressWarnings("unchecked")
    // unsafe, comparator() returns a comparator on the specified type
    // TODO(kevinb): evaluate whether or not of().comparator() should return null
    public static <K, V> ImmutableSortedMap<K, V> of() {
      return (ImmutableSortedMap<K, V>) NATURAL_EMPTY_MAP;
    }
docs: Returns the empty sorted map.