code: func (idx *InvertedIndex) Add(docs ...Document) error {
  	var err error
  	for _, config := range idx.cfg.Indices {
  		if config.Weight == 0 {
  			// Disabled
  			continue
  		}
  		setm := idx.index[config.Name]
  
  		for _, doc := range docs {
  			var words []Keyword
  			words, err = doc.RelatedKeywords(config)
  			if err != nil {
  				continue
  			}
  
  			for _, keyword := range words {
  				setm[keyword] = append(setm[keyword], doc)
  			}
  		}
  	}
  
  	return err
  
  }
docs: // Add documents to the inverted index.
  // The value must support == and !=.