docs: @SuppressWarnings("ConstantConditions")
    ImmutableListMultimap<K,V> toImmutableListMultimap() {
      final ImmutableListMultimap.Builder<K, V>  ret = ImmutableListMultimap.builder();
  
      for (final MultimapEntry<K, V> entry : entries) {
        ret.putAll(entry.key, entry.values);
      }
  
      return ret.build();
    }