<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def export_cfg(obj, file_name):
 """ Exports curves and surfaces in libconfig format. .. note:: Requires `libconf <https://pypi.org/project/libconf/>`_ package. Libconfig format is also used by the `geomdl command-line application <https://github.com/orbingol/geomdl-cli>`_ as a way to input shape data from the command line. :param obj: input geometry :type obj: abstract.SplineGeometry, multi.AbstractContainer :param file_name: name of the output file :type file_name: str :raises GeomdlException: an error occurred writing the file """

    def callback(data):
        return libconf.dumps(data)

    # Check if it is possible to import 'libconf'
    try:
        import libconf
    except ImportError:
        raise exch.GeomdlException("Please install 'libconf' package to use libconfig format: pip install libconf")

    # Export data
    exported_data = exch.export_dict_str(obj=obj, callback=callback)

    # Write to file
    return exch.write_file(file_name, exported_data)