docs: def _add_inserter(self):
          """
          Add an ``_insert_x()`` method to the element class for this child
          element.
          """
          def _insert_child(obj, child):
              obj.insert_element_before(child, *self._successors)
              return child
  
          _insert_child.__doc__ = (
              'Return the passed ``<%s>`` element after inserting it as a chil'
              'd in the correct sequence.' % self._nsptagname
          )
          self._add_to_class(self._insert_method_name, _insert_child)