Returns: next free Unit ID

def next_unit_id(self) -> int:
        """
        Returns: next free Unit ID
        """
        ids: typing.Set[int] = set()
        for unit in chain(self._blue_coa.units, self._red_coa.units):  # type: ignore

            id_ = unit.unit_id
            if id_ in ids:
                raise IndexError(unit.unit_name)
            ids.add(id_)
        return max(ids) + 1