cal is treated as a ordered categorical.
        None can be used to maintain the ordered value of existing categoricals when
        used in operations that combine categoricals, e.g. astype, and will resolve to
        False if there is no existing ordered to maintain.

    Attributes
    ----------
    categories
    ordered

    Methods
    -------
    None

    See Also
    --------
    Categorical : Represent a categorical variable in classic R / S-plus fashion.

    Notes
    -----
    This class is useful for specifying the type of a ``Categorical``
    independent of the values. See :ref:`categorical.categoricaldtype`
    for more.

    Examples
    --------
    >>> t = pd.CategoricalDtype(categories=['b', 'a'], ordered=True)
    >>> pd.Series(['a', 'b', 'a', 'c'], dtype=t)
    0      a
    1      b
    2      a
    3    NaN
    dtype: category
    Categories (2, object): ['b' < 'a']

    An empty CategoricalDtype with a specific dtype can be created
    by providing an empty index. As follows,

    >>> pd.CategoricalDtype(pd.DatetimeIndex([])).categories.dtype
    dtype('<M8[ns]')
    Úcategoryz