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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 doc2vec(self, doc):
 """Convert a single SimilarityDocument to vector."""

        bow = self.dictionary.doc2bow(doc['tokens'])
        if self.method == 'lsi':
            return self.lsi[self.tfidf[bow]]
        elif self.method == 'lda':
            return self.lda[bow]
        elif self.method == 'lda_tfidf':
            return self.lda[self.tfidf[bow]]
        elif self.method == 'logentropy':
            return self.logent[bow]