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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 crs(self, crs):
 """Setter for extent_crs property. :param crs: The coordinate reference system for the analysis boundary. :type crs: QgsCoordinateReferenceSystem """

        if isinstance(crs, QgsCoordinateReferenceSystem):
            self._crs = crs
            self._is_ready = False
        else:
            raise InvalidExtentError('%s is not a valid CRS object.' % crs)