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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 initialize(cls) -> None: """Initializes the ``SIGCHLD`` handler. The signal handler is run on an `.IOLoop` to avoid locking issues. Note that the `.IOLoop` used for signal handling need not be the same one used by individual Subprocess objects (as long as the ``IOLoops`` are each running in separate threads). .. versionchanged:: 5.0 The ``io_loop`` argument (deprecated since version 4.1) has been removed. Availability: Unix """

        if cls._initialized:
            return
        io_loop = ioloop.IOLoop.current()
        cls._old_sigchld = signal.signal(
            signal.SIGCHLD,
            lambda sig, frame: io_loop.add_callback_from_signal(cls._cleanup),
        )
        cls._initialized = True