code: def prob_classify(self, text):
          
          feats = self.extract_features(text)
          return self.classifier.prob_classify(feats)
docs: Return the label probability distribution for classifying a string
          of text.
  
          Example:
          ::
  
              >>> classifier = MaxEntClassifier(train_data)
              >>> prob_dist = classifier.prob_classify("I feel happy this morning.")
              >>> prob_dist.max()
              'positive'
              >>> prob_dist.prob("positive")
              0.7
  
          :rtype: nltk.probability.DictionaryProbDist