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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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]