docs: def add_how(voevent, descriptions=None, references=None):
      """Add descriptions or references to the How section.
  
      Args:
          voevent(:class:`Voevent`): Root node of a VOEvent etree.
          descriptions(str): Description string, or list of description
              strings.
          references(:py:class:`voeventparse.misc.Reference`): A reference element
              (or list thereof).
      """
      if not voevent.xpath('How'):
          etree.SubElement(voevent, 'How')
      if descriptions is not None:
          for desc in _listify(descriptions):
              # d = etree.SubElement(voevent.How, 'Description')
              # voevent.How.Description[voevent.How.index(d)] = desc
              ##Simpler:
              etree.SubElement(voevent.How, 'Description')
              voevent.How.Description[-1] = desc
      if references is not None:
          voevent.How.extend(_listify(references))