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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 _keys_to_lower(self):
 """Convert key set to lowercase."""

        for k in list(self.keys()):
            val = super(CaseInsensitiveDict, self).__getitem__(k)
            super(CaseInsensitiveDict, self).__delitem__(k)
            self.__setitem__(CaseInsensitiveStr(k), val)