docs: def set_attribute(self, aid, attrib, val, idx='*'):
          """
          Set the value of an xml attribute marked with the matching aid attribute.
          """
          if aid in self.__attrib_ids:
              elems = self.__attrib_ids[aid]
              if idx == '*':
                  for elem in elems:
                      self.__set_attribute(elem, attrib, val)
              elif idx < len(elems):
                  elem = elems[idx]
                  self.__set_attribute(elem, attrib, val)