docs: def sheets(self, index=None):
      """Return either a list of all sheets if index is None, or the sheet at the given index."""
      if self._sheets is None:
        self._sheets = [self.get_worksheet(s, i) for i, s in enumerate(self.iterate_sheets())]
      if index is None:
        return self._sheets
      else:
        return self._sheets[index]