code: func (ss *IndexSearcher) explain(weight Weight, doc int) (exp Explanation, err error) {
  	n := index.SubIndex(doc, ss.leafContexts)
  	ctx := ss.leafContexts[n]
  	deBasedDoc := doc - ctx.DocBase
  	return weight.Explain(ctx, deBasedDoc)
  }
docs: /*
  Expert: low-level implementation method
  Returns an Explanation that describes how doc scored against weight.
  
  This is intended to be used in developing Similarity implementations, 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.
  
  Applications should call explain(Query, int).
  */