code: func ReadHeaderNumber(db ethdb.Reader, hash common.Hash) *uint64 {
  	data, _ := db.Get(headerNumberKey(hash))
  	if len(data) != 8 {
  		return nil
  	}
  	number := binary.BigEndian.Uint64(data)
  	return &number
  }
docs: // ReadHeaderNumber returns the header number assigned to a hash.