code: func ReadCanonicalHash(db ethdb.Reader, number uint64) common.Hash {
  	data, _ := db.Get(headerHashKey(number))
  	if len(data) == 0 {
  		return common.Hash{}
  	}
  	return common.BytesToHash(data)
  }
docs: // ReadCanonicalHash retrieves the hash assigned to a canonical block number.