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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 fit(self, X, y):
 """Learn vocabulary from training set. Args: X : iterable. An iterable which yields either str, unicode or file objects. Returns: self : IndexTransformer. """

        self._word_vocab.add_documents(X)
        self._label_vocab.add_documents(y)
        if self._use_char:
            for doc in X:
                self._char_vocab.add_documents(doc)

        self._word_vocab.build()
        self._char_vocab.build()
        self._label_vocab.build()

        return self