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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 to_string(self, format_, fps=None, **kwargs):
 """ Get subtitle file as a string. See :meth:`SSAFile.save()` for full description. Returns: str """

        fp = io.StringIO()
        self.to_file(fp, format_, fps=fps, **kwargs)
        return fp.getvalue()