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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 clear(self):
 """Close all opened files if not in use."""

        with self.lock:
            for filehandle, refcount in list(self.files.items()):
                if refcount == 0:
                    filehandle.close()
                    del self.files[filehandle]
                    del self.past[self.past.index(filehandle)]