code: func (ss *IndexSearcher) Explain(query Query, doc int) (exp Explanation, err error) {
  	w, err := ss.spi.CreateNormalizedWeight(query)
  	if err == nil {
  		return ss.explain(w, doc)
  	}
  	return
  }
docs: /*
  Returns an Explanation that describes how doc scored against query.
  
  This is intended to be used in developing Similiarity implemenations, and, for
  good performance, should not be displayed with every hit. Computing an
  explanation is as expensive as executing the query over the entire index.
  */