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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_document_unit(self):
 """Get the unit of the SVG surface. If the surface passed as an argument is not a SVG surface, the function sets the error status to ``STATUS_SURFACE_TYPE_MISMATCH`` and returns :ref:`SVG_UNIT_USER`. :return: The SVG unit of the SVG surface. *New in cairo 1.16.* *New in cairocffi 0.9.* """

        unit = cairo.cairo_svg_surface_get_document_unit(self._pointer)
        self._check_status()
        return unit