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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 sheets(self):
 """ Collection of sheets in this document. """

        # http://www.openoffice.org/api/docs/common/ref/com/sun/star/sheet/XSpreadsheetDocument.html#getSheets
        try:
            return self._sheets
        except AttributeError:
            target = self._target.getSheets()
            self._sheets = SpreadsheetCollection(self, target)
        return self._sheets