Enters stem as value for source.

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())