code: func (c *TermVectorsConsumer) fill(docId int) error {
  	for c.lastDocId < docId {
  		c.writer.StartDocument(0)
  		err := c.writer.FinishDocument()
  		if err != nil {
  			return err
  		}
  		c.lastDocId++
  	}
  	return nil
  }
docs: /*
  Fills in no-term-vectors for all docs we haven't seen since the last
  doc that had term vectors.
  */