def _export_coord_axes(nodes, bars, csv_filename: str):
    """save the coordinate systems in a csv file"""
    if csv_filename:
        with open(csv_filename, 'w') as out_file:
            for node in nodes:
                out_file.write('GRID,%i,%s,%s,%s\n' % node)
            for bari in bars:
                out_file.write('BAR,%i,%i,%i\n' % bari)
