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