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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 pos_tag(sentence, format=None):
 """ Vietnamese POS tagging Parameters ========== sentence: {unicode, str} Raw sentence Returns ======= tokens: list of tuple with word, pos tag tagged sentence Examples -------- [('Chợ', 'N'), ('thịt', 'N'), ('chó', 'N'), ('nổi tiếng', 'A'), ('ở', 'E'), ('TPHCM', 'Np'), ('bị', 'V'), ('truy quét', 'V')] """

    sentence = word_tokenize(sentence)
    crf_model = CRFPOSTagPredictor.Instance()
    result = crf_model.predict(sentence, format)
    return result