code: func (hc *HeaderChain) GetTdByHash(hash common.Hash) *big.Int {
  	number := hc.GetBlockNumber(hash)
  	if number == nil {
  		return nil
  	}
  	return hc.GetTd(hash, *number)
  }
docs: // GetTdByHash retrieves a block's total difficulty in the canonical chain from the
  // database by hash, caching it if found.