docs: def required_elements(element, *children):
      """Check element (``xml.etree.ElementTree.Element``) for required children, defined as XPath. Raise
      ``NotValidXmlException`` on error.
  
      :param element: ElementTree element
      :param children: list of XPaths to check
      :raises NotValidXmlException: if some child is missing
      """
      for child in children:
          if element.find(child) is None:
              raise NotValidXmlException(msg_err_missing_children(element.tag, *children))