docs: def chunks(self):
          """Block dimensions for this dataset's data or None if it's not a dask
          array.
          """
          chunks = {}
          for v in self.variables.values():
              if v.chunks is not None:
                  for dim, c in zip(v.dims, v.chunks):
                      if dim in chunks and c != chunks[dim]:
                          raise ValueError('inconsistent chunks')
                      chunks[dim] = c
          return Frozen(SortedKeysDict(chunks))