code: def _indent_xml(xml, prefix=''):
    
    if len(xml) > 0:
      xml.text = (xml.text or '').strip() + '\n' + prefix + '  '
      for child in xml:
        _indent_xml(child, prefix+'  ')
      for child in xml[:-1]:
        child.tail = (child.tail or '').strip() + '\n' + prefix + '  '
      xml[-1].tail = (xml[-1].tail or '').strip() + '\n' + prefix
docs: Helper for ``build_index()``: Given an XML ``ElementTree``, modify it
    (and its descendents) ``text`` and ``tail`` attributes to generate
    an indented tree, where each nested element is indented by 2
    spaces with respect to its parent.