code: def sum(self, values, axis=0, dtype=None):
          
          values = np.asarray(values)
          return self.unique, self.reduce(values, axis=axis, dtype=dtype)
docs: compute the sum over each group
  
          Parameters
          ----------
          values : array_like, [keys, ...]
              values to sum per group
          axis : int, optional
              alternative reduction axis for values
          dtype : output dtype
  
          Returns
          -------
          unique: ndarray, [groups]
              unique keys
          reduced : ndarray, [groups, ...]
              value array, reduced over groups