@classmethod
def add_root(cls, **kwargs):
    """
        Adds a root node to the tree. The new root node will be the new
        rightmost root node. If you want to insert a root node at a specific
        position, use :meth:`add_sibling` in an already existing root node
        instead.

        :param \\*\\*kwargs: object creation data that will be passed to the
            inherited Node model
        :param instance: Instead of passing object creation data, you can
            pass an already-constructed (but not yet saved) model instance to
            be inserted into the tree.

        :returns: the created node object. It will be save()d by this method.

        :raise NodeAlreadySaved: when the passed ``instance`` already exists
            in the database
        """
    raise NotImplementedError
