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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_list(self, data):
        'Flattens nested lists into strings.'

        if data is None:
            return ''
        if isinstance(data, types.StringTypes):
            return data
        elif isinstance(data, (list, tuple)):
            return '\n'.join(self._flatten_list(x) for x in data)