code: def stop():
      
      global global_server
      if global_server is not None:
          ioloop = global_server.ioloop
          def stop_ioloop():
              ioloop.stop()
              ioloop.close()
          global_server.ioloop.add_callback(stop_ioloop)
          global_server = None
docs: Stop the server, invalidating any viewer URLs.
  
      This allows any previously-referenced data arrays to be garbage collected if there are no other
      references to them.