docs: def EventIvorn(ivorn, cite_type):
      """
      Used to cite earlier VOEvents.
  
      Use in conjunction with :func:`.add_citations`
  
      Args:
          ivorn(str): It is assumed this will be copied verbatim from elsewhere,
              and so these should have any prefix (e.g. 'ivo://','http://')
              already in place - the function will not alter the value.
          cite_type (:class:`.definitions.cite_types`): String conforming to one
              of the standard citation types.
  
      """
      # This is an ugly hack around the limitations of the  lxml.objectify API:
      c = objectify.StringElement(cite=cite_type)
      c._setText(ivorn)
      c.tag = "EventIVORN"
      return c