docs: def _nonzero(self):
          """ Equivalent numpy's nonzero but returns a tuple of Varibles. """
          # TODO we should replace dask's native nonzero
          # after https://github.com/dask/dask/issues/1076 is implemented.
          nonzeros = np.nonzero(self.data)
          return tuple(Variable((dim), nz) for nz, dim
                       in zip(nonzeros, self.dims))