code: def reduce(self, func, **kwargs):
          
          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)
docs: 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.