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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 setup(channel, direction, initial=None, pull_up_down=None):
 """ You need to set up every channel you are using as an input or an output. :param channel: the channel based on the numbering system you have specified (:py:attr:`GPIO.BOARD`, :py:attr:`GPIO.BCM` or :py:attr:`GPIO.SUNXI`). :param direction: whether to treat the GPIO pin as input or output (use only :py:attr:`GPIO.IN` or :py:attr:`GPIO.OUT`). :param initial: (optional) When supplied and setting up an output pin, resets the pin to the value given (can be :py:attr:`0` / :py:attr:`GPIO.LOW` / :py:attr:`False` or :py:attr:`1` / :py:attr:`GPIO.HIGH` / :py:attr:`True`). :param pull_up_down: (optional) When supplied and setting up an input pin, configures the pin to 3.3V (pull-up) or 0V (pull-down) depending on the value given (can be :py:attr:`GPIO.PUD_OFF` / :py:attr:`GPIO.PUD_UP` / :py:attr:`GPIO.PUD_DOWN`) To configure a channel as an input: .. code:: python GPIO.setup(channel, GPIO.IN) To set up a channel as an output: .. code:: python GPIO.setup(channel, GPIO.OUT) You can also specify an initial value for your output channel: .. code:: python GPIO.setup(channel, GPIO.OUT, initial=GPIO.HIGH) **Setup more than one channel:** You can set up more than one channel per call. For example: .. code:: python chan_list = [11,12] # add as many channels as you want! # you can tuples instead i.e.: # chan_list = (11,12) GPIO.setup(chan_list, GPIO.OUT) """

    if _mode is None:
        raise RuntimeError("Mode has not been set")

    if pull_up_down is not None:
        if _gpio_warnings:
            warnings.warn("Pull up/down setting are not (yet) fully supported, continuing anyway. Use GPIO.setwarnings(False) to disable warnings.", stacklevel=2)

    if isinstance(channel, list):
        for ch in channel:
            setup(ch, direction, initial)
    else:
        if channel in _exports:
            raise RuntimeError("Channel {0} is already configured".format(channel))
        pin = get_gpio_pin(_mode, channel)
        try:
            sysfs.export(pin)
        except (OSError, IOError) as e:
            if e.errno == 16:   # Device or resource busy
                if _gpio_warnings:
                    warnings.warn("Channel {0} is already in use, continuing anyway. Use GPIO.setwarnings(False) to disable warnings.".format(channel), stacklevel=2)
                sysfs.unexport(pin)
                sysfs.export(pin)
            else:
                raise e

        sysfs.direction(pin, direction)
        _exports[channel] = direction
        if direction == OUT and initial is not None:
            sysfs.output(pin, initial)