Returns the GeoQuerySet extent as a 4-tuple.

        Keyword args:
        srid -- EPSG id for for transforming the output geometry.

def extent(self, srid=None):
        """Returns the GeoQuerySet extent as a 4-tuple.

        Keyword args:
        srid -- EPSG id for for transforming the output geometry.
        """
        expr = self.geo_field.name
        if srid:
            expr = geofn.Transform(expr, srid)
        expr = models.Extent(expr)
        clone = self.all()
        name, val = clone.aggregate(expr).popitem()
        return val