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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 filenames(self):
 """A list of all handled auxiliary file names. ['file1', 'file2'] """

        fns = set()
        for fn2var in self._type2filename2variable.values():
            fns.update(fn2var.keys())
        return sorted(fns)