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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 add_citations(voevent, event_ivorns):
 """Add citations to other voevents. The schema mandates that the 'Citations' section must either be entirely absent, or non-empty - hence we require this wrapper function for its creation prior to listing the first citation. Args: voevent(:class:`Voevent`):
 Root node of a VOEvent etree. event_ivorns (:class:`voeventparse.misc.EventIvorn`):
 List of EventIvorn elements to add to citation list. """

    if not voevent.xpath('Citations'):
        etree.SubElement(voevent, 'Citations')
    voevent.Citations.extend(_listify(event_ivorns))