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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _group_events(catalog, process_len, template_length, data_pad):
 """ Internal function to group events into sub-catalogs based on process_len. :param catalog: Catalog to groups into sub-catalogs :type catalog: obspy.core.event.Catalog :param process_len: Length in seconds that data will be processed in :type process_len: int :return: List of catalogs :rtype: list """

    # case for catalog only containing one event
    if len(catalog) == 1:
        return [catalog]
    sub_catalogs = []
    # Sort catalog by date
    catalog.events = sorted(
        catalog.events,
        key=lambda e: (e.preferred_origin() or e.origins[0]).time)
    sub_catalog = Catalog([catalog[0]])
    for event in catalog[1:]:
        origin_time = (event.preferred_origin() or event.origins[0]).time
        last_pick = sorted(event.picks, key=lambda p: p.time)[-1]
        max_diff = (
            process_len - (last_pick.time - origin_time) - template_length)
        max_diff -= 2 * data_pad
        if origin_time - sub_catalog[0].origins[0].time < max_diff:
            sub_catalog.append(event)
        else:
            sub_catalogs.append(sub_catalog)
            sub_catalog = Catalog([event])
    sub_catalogs.append(sub_catalog)
    return sub_catalogs