docs: def update_elements(dst_pyxb, src_pyxb, el_list):
      """Copy elements specified in ``el_list`` from ``src_pyxb`` to ``dst_pyxb``
  
      Only elements that are children of root are supported. See
      SYSMETA_ROOT_CHILD_LIST.
  
      If an element in ``el_list`` does not exist in ``src_pyxb``, it is removed from
      ``dst_pyxb``.
  
      """
      invalid_element_set = set(el_list) - set(SYSMETA_ROOT_CHILD_LIST)
      if invalid_element_set:
          raise ValueError(
              'Passed one or more invalid elements. invalid="{}"'.format(
                  ', '.join(sorted(list(invalid_element_set)))
              )
          )
      for el_str in el_list:
          setattr(dst_pyxb, el_str, getattr(src_pyxb, el_str, None))