docs: 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)