How to use the pscript.stubs.window.flexx.spin function in pscript

def _receive_command(self, command):
        """ Process a command send from the server.
        """
        cmd = command[0]
        if cmd == 'PING':
            # Used for roundtrip stuff, do at least one iter loop here ...
            window.setTimeout(self.send_command, 10, 'PONG', command[1])
        elif cmd == 'INIT_DONE':
            window.flexx.spin(None)
            while len(self._pending_commands):
                self._receive_raw_command(self._pending_commands.pop(0))
            self._pending_commands = None
            # print('init took', time() - self._init_time)
        elif cmd == 'PRINT':
            (window.console.ori_log or window.console.log)(command[1])
        elif cmd == 'EXEC':
            eval(command[1])
        elif cmd == 'EVAL':
            x = None
            if len(command) == 2:
                x = eval(command[1])
            elif len(command) == 3:
                x = eval('this.instances.' + command[1] + '.' + command[2])
            console.log(str(x))  # print (and thus also sends back result)
        elif cmd == 'EVALANDRETURN':