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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 _addPub(self, stem, source):
 """Enters stem as value for source. """

        key = re.sub("[^A-Za-z0-9&]+", " ", source).strip().upper()
        self.sourceDict[key] = stem
        self.bibstemWords.setdefault(stem, set()).update(
            key.lower().split())