code: func Checksum(address Hash) (Trytes, error) {
  	if len(address) < 81 {
  		return "", ErrInvalidAddress
  	}
  
  	addressWithChecksum, err := checksum.AddChecksum(address[:81], true, 9)
  	if err != nil {
  		return "", err
  	}
  	return addressWithChecksum[AddressWithChecksumTrytesSize-AddressChecksumTrytesSize : 90], nil
  }
docs: // Checksum returns the checksum of the given address.