docs: 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))