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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 preprocess_plain_text_file(self, filename, pmid, extra_annotations):
 """Preprocess a plain text file for use with ISI reder. Preprocessing results in a new text file with one sentence per line. Parameters filename : str The name of the plain text file pmid : str The PMID from which it comes, or None if not specified extra_annotations : dict Extra annotations to be added to each statement, possibly including metadata about the source (annotations with the key "interaction" will be overridden) """

        with codecs.open(filename, 'r', encoding='utf-8') as f:
            content = f.read()
            self.preprocess_plain_text_string(content, pmid,
                                              extra_annotations)