Normalize this node prior to validation. This is required, as the
        ogc:Filter node must come before any symbolizer nodes. The SLD
        is modified in place.

def normalize(self):
        """
        Normalize this node prior to validation. This is required, as the
        ogc:Filter node must come before any symbolizer nodes. The SLD
        is modified in place.
        """
        order = [
            'sld:Title', 'ogc:Filter', 'sld:MinScaleDenominator',
            'sld:MaxScaleDenominator', 'sld:PolygonSymbolizer',
            'sld:LineSymbolizer', 'sld:TextSymbolizer', 'sld:PointSymbolizer']
        for item in order:
            xpath = self._node.xpath(item, namespaces=SLDNode._nsmap)
            for xitem in xpath:
                # move this to the end
                self._node.remove(xitem)
                self._node.append(xitem)