docs: def _get_startstop(sheet, startcell=None, stopcell=None):
      """
      Return two StartStop objects, based on the sheet and startcell and
      stopcell.
  
      sheet: xlrd.sheet.Sheet instance
          Ready for use.
  
      startcell: str or None
          If given, a spread sheet style notation of the cell where data
          start, ("F9").
  
      stopcell: str or None
          A spread sheet style notation of the cell where data end,
          ("F9"). startcell and stopcell can be used in any combination.
  
      """
  
      start = StartStop(0, 0)     # row, col
      stop = StartStop(sheet.nrows, sheet.ncols)
  
      if startcell:
          m = re.match(XLNOT_RX, startcell)
          start.row = int(m.group(2)) - 1
          start.col = letter2num(m.group(1), zbase=True)
  
      if stopcell:
          m = re.match(XLNOT_RX, stopcell)
          stop.row = int(m.group(2))
          # Stop number is exclusive
          stop.col = letter2num(m.group(1), zbase=False)
  
      return [start, stop]