docs: def cross_validation_lock(obj):
      """
      A contextmanager for holding Traited object's cross-validators.
  
      This should be used in circumstances where you want to call _validate, but
      don't want to fire cross-validators.
      """
      # TODO: Replace this with usage of public API when
      # https://github.com/ipython/traitlets/pull/166 lands upstream.
      orig = getattr(obj, '_cross_validation_lock', False)
      try:
          obj._cross_validation_lock = True
          yield
      finally:
          obj._cross_validation_lock = orig