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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 groupletters(x, _low=lower_function):
 """ Double all characters, making doubled letters lowercase. Parameters x : str Returns ------- str Examples -------- {u}'aAppppllee' """

    return "".join(ichain.from_iterable((_low(y), y) for y in x))