docs: def get_class_xsd(self, io, cls):
          """ Returns the XSD for a mapped class. """
          attrs = {}
          attrs['xmlns:gml'] = 'http://www.opengis.net/gml'
          attrs['xmlns:xsd'] = 'http://www.w3.org/2001/XMLSchema'
          tb = TreeBuilder()
          with tag(tb, 'xsd:schema', attrs) as tb:
              with tag(tb, 'xsd:complexType', {'name': cls.__name__}) as tb:
                  with tag(tb, 'xsd:complexContent') as tb:
                      with tag(tb, 'xsd:extension',
                               {'base': 'gml:AbstractFeatureType'}) as tb:
                          with tag(tb, 'xsd:sequence') as tb:
                              self.add_class_properties_xsd(tb, cls)
  
          ElementTree(tb.close()).write(io, encoding='utf-8')
          return io