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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 _listChunks(self):
 """ Lists stored chunks :return: sorted list of available chunk indices """

        chunks = []
        for fileName in os.listdir(self.dataDir):
            index = ChunkedFileStore._fileNameToChunkIndex(fileName)
            if index is not None:
                chunks.append(index)
        return sorted(chunks)