code: func (hc *HeaderChain) WriteTd(hash common.Hash, number uint64, td *big.Int) error {
  	rawdb.WriteTd(hc.chainDb, hash, number, td)
  	hc.tdCache.Add(hash, new(big.Int).Set(td))
  	return nil
  }
docs: // WriteTd stores a block's total difficulty into the database, also caching it
  // along the way.