def unit_system_id(self):
        """
        This is a unique identifier for the unit registry created
        from a FNV hash. It is needed to register a dataset's code
        unit system in the unit system registry.
        """
        if self._unit_system_id is None:
            hash_data = bytearray()
            for k, v in sorted(self.lut.items()):
                hash_data.extend(k.encode("utf8"))
                hash_data.extend(repr(v).encode("utf8"))
            m = md5()
            m.update(hash_data)
            self._unit_system_id = str(m.hexdigest())
        return self._unit_system_id
This is a unique identifier for the unit registry created
        from a FNV hash. It is needed to register a dataset's code
        unit system in the unit system registry.