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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 style_sheet(self):
 """ Access the style_sheet :returns: twilio.rest.autopilot.v1.assistant.style_sheet.StyleSheetList :rtype: twilio.rest.autopilot.v1.assistant.style_sheet.StyleSheetList """

        if self._style_sheet is None:
            self._style_sheet = StyleSheetList(self._version, assistant_sid=self._solution['sid'], )
        return self._style_sheet