docs: def create_closure_model(cls):
      """Creates a <Model>Closure model in the same module as the model."""
      meta_vals = {
          'unique_together':  (("parent", "child"),)
      }
      if getattr(cls._meta, 'db_table', None):
          meta_vals['db_table'] = '%sclosure' % getattr(cls._meta, 'db_table')
      model = type('%sClosure' % cls.__name__, (models.Model,), {
          'parent': models.ForeignKey(
              cls.__name__,
              related_name=cls.closure_parentref()
          ),
          'child': models.ForeignKey(
              cls.__name__,
              related_name=cls.closure_childref()
          ),
          'depth': models.IntegerField(),
          '__module__':   cls.__module__,
          '__unicode__': _closure_model_unicode,
          'Meta': type('Meta', (object,), meta_vals),
      })
      setattr(cls, "_closure_model", model)
      return model