input: Write a function to analyze the sentiment of given text and categorize it into positive, negative, and neutral sentiments. The function should take a string of text as input and return a string indicating the sentiment. If the method for sentiment analysis is not defined, assuming it has been defined already as `NaiveBayesClassifier.classify` returning 'pos', 'neg', or none.
  assert analyze_sentiment("I am happy")=="positive"
  assert analyze_sentiment("I am sad")=="negative"
  assert analyze_sentiment("Today is a good day, but yesterday was not")=="neutral"
  assert analyze_sentiment("")=="neutral"
  assert analyze_sentiment("The movie was excellent, the worst I've ever seen")=="negative"
output: def analyze_sentiment(text):
      sentiment = NaiveBayesClassifier.classify(text)
      if sentiment == 'pos':
          return "positive"
      elif sentiment == 'neg':
          return "negative"
      else:
          return "neutral"