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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 split_sentences(self):
 """ ROUGE requires texts split into sentences. In case the texts are not already split, this method can be used. """

        from pyrouge.utils.sentence_splitter import PunktSentenceSplitter
        self.log.info("Splitting sentences.")
        ss = PunktSentenceSplitter()
        sent_split_to_string = lambda s: "\n".join(ss.split(s))
        process_func = partial(
            DirectoryProcessor.process, function=sent_split_to_string)
        self.__process_summaries(process_func)