code: @Override 
    @GwtIncompatible // NavigableMap
    public NavigableMap<K, Collection<V>> asMap() {
      return (NavigableMap<K, Collection<V>>) super.asMap();
    }
docs: {@inheritDoc}
  
  <p>Because a {@code TreeMultimap} has unique sorted keys, this method
  returns a {@link NavigableMap}, instead of the {@link java.util.Map} specified
  in the {@link Multimap} interface.
  
  @since 14.0 (present with return type {@code SortedMap} since 2.0)