@staticmethod
def _idgen_sources(group_id, subject_number):
    """
        Identifiers generator
        :param gid: group ID
        :param subn: subject ID
        :return: str
        """
    idarr = []
    if group_id != '':
        idarr.append('GRP-{}'.format(group_id))
    if subject_number != '':
        idarr.append('SBJ-{}'.format(subject_number))
    return '.'.join(idarr).replace(' ', '_')
