Return grouped, sorted namedtuples in either format: flat, sections.

def get_fields(desc2nts):
        """Return grouped, sorted namedtuples in either format: flat, sections."""
        if 'flat' in desc2nts:
            nts_flat = desc2nts.get('flat')
            if nts_flat:
                return nts_flat[0]._fields
        if 'sections' in desc2nts:
            nts_sections = desc2nts.get('sections')
            if nts_sections:
                return nts_sections[0][1][0]._fields