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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 flatten(nested):
 """ Return a flatten version of the nested argument """

    flat_return = list()

    def __inner_flat(nested,flat):
        for i in nested:
            __inner_flat(i, flat) if isinstance(i, list) else flat.append(i)
        return flat

    __inner_flat(nested,flat_return)

    return flat_return