Reduce the items in this group by applying `func` along some
        dimension(s).

        Parameters
        ----------
        func : function
            Function which can be called in the form
            `func(x, **kwargs)` to return the result of collapsing an
            np.ndarray over an the rolling dimension.
        **kwargs : dict
            Additional keyword arguments passed on to `func`.

        Returns
        -------
        reduced : DataArray
            Array with summarized data.

def reduce(self, func, **kwargs):
        """Reduce the items in this group by applying `func` along some
        dimension(s).

        Parameters
        ----------
        func : function
            Function which can be called in the form
            `func(x, **kwargs)` to return the result of collapsing an
            np.ndarray over an the rolling dimension.
        **kwargs : dict
            Additional keyword arguments passed on to `func`.

        Returns
        -------
        reduced : DataArray
            Array with summarized data.
        """
        from .dataset import Dataset
        reduced = OrderedDict()
        for key, da in self.obj.data_vars.items():
            if self.dim in da.dims:
                reduced[key] = self.rollings[key].reduce(func, **kwargs)
            else:
                reduced[key] = self.obj[key]
        return Dataset(reduced, coords=self.obj.coords)