code: func ReadTd(db ethdb.Reader, hash common.Hash, number uint64) *big.Int {
  	data := ReadTdRLP(db, hash, number)
  	if len(data) == 0 {
  		return nil
  	}
  	td := new(big.Int)
  	if err := rlp.Decode(bytes.NewReader(data), td); err != nil {
  		log.Error("Invalid block total difficulty RLP", "hash", hash, "err", err)
  		return nil
  	}
  	return td
  }
docs: // ReadTd retrieves a block's total difficulty corresponding to the hash.