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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 key_of(d):
 """ Returns the key of a single element dict. """

    if len(d) > 1 and not type(d) == dict():
        raise ValueError('key_of(d) may only except single element dict')
    else:
        return keys_of(d)[0]