docs: def _add_adder(self):
          """
          Add an ``_add_x()`` method to the element class for this child
          element.
          """
          def _add_child(obj, **attrs):
              new_method = getattr(obj, self._new_method_name)
              child = new_method()
              for key, value in attrs.items():
                  setattr(child, key, value)
              insert_method = getattr(obj, self._insert_method_name)
              insert_method(child)
              return child
  
          _add_child.__doc__ = (
              'Add a new ``<%s>`` child element unconditionally, inserted in t'
              'he correct sequence.' % self._nsptagname
          )
          self._add_to_class(self._add_method_name, _add_child)