docs: def _getitem(self, key):
          """Return specified page of series from cache or file."""
          key = int(key)
          if key < 0:
              key %= self._len
          if len(self._pages) == 1 and 0 < key < self._len:
              index = self._pages[0].index
              return self.parent.pages._getitem(index + key)
          return self._pages[key]