Create an empty dicttree.

        The root node has a special attribute "_rootname".
        Because root node is the only dictionary doesn't have key. 
        So we assign the key as a special attribute.

        Usage::

            >>> from weatherlab.lib.dtypes.dicttree import DictTree as DT
            >>> d = DT.initial("US)
            >>> d
            {'_meta': {'_rootname': 'US'}}

def initial(key, **kwarg):
        """Create an empty dicttree.

        The root node has a special attribute "_rootname".
        Because root node is the only dictionary doesn't have key. 
        So we assign the key as a special attribute.

        Usage::

            >>> from weatherlab.lib.dtypes.dicttree import DictTree as DT
            >>> d = DT.initial("US)
            >>> d
            {'_meta': {'_rootname': 'US'}}
        """
        d = dict()
        DictTree.setattr(d, _rootname = key, **kwarg)
        return d