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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_socket(self, socket):
 """ Add a socket to the multiplexer. :param socket: The socket. If it was added already, it won't be added a second time. """

        if socket not in self._sockets:
            self._sockets.add(socket)
            socket.on_closed.connect(self.remove_socket)