code: func (c *Conn) Explain(index string, _type string, id string, args map[string]interface{}, query string) (Match, error) {
  	var url string
  	var retval Match
  	if len(_type) > 0 {
  		url = fmt.Sprintf("/%s/%s/_explain", index, _type)
  	} else {
  		url = fmt.Sprintf("/%s/_explain", index)
  	}
  	body, err := c.DoCommand("GET", url, args, query)
  	if err != nil {
  		return retval, err
  	}
  	if err == nil {
  		// marshall into json
  		jsonErr := json.Unmarshal(body, &retval)
  		if jsonErr != nil {
  			return retval, jsonErr
  		}
  	}
  	return retval, err
  }
docs: // Explain computes a score explanation for a query and a specific document.
  // This can give useful feedback whether a document matches or didn’t match a specific query.
  // This feature is available from version 0.19.9 and up.
  // see http://www.elasticsearch.org/guide/reference/api/explain.html