input: Write a function to identify whether a given sentence is an 'affirmative' sentence or a 'negative' sentence. The function should use the NLTK library to identify negation words in the sentence. If no negation words are found, the sentence is considered 'affirmative'; otherwise, it is considered 'negative'. If the sentence is empty, it should be considered 'affirmative'.
  assert isAffirmativeorNegative("He always comes late but I don't mind.")=="Negative"
  assert isAffirmativeorNegative("She is happy.")=="Affirmative"
  assert isAffirmativeorNegative("No one was there.")=="Negative"
  assert isAffirmativeorNegative("They will never come.")=="Negative"
  assert isAffirmativeorNegative("")=="Affirmative"
output: def isAffirmativeorNegative(sentence):
      negation_words = {"not", "no", "never", "none", "nobody", "nothing", "neither", "nowhere", "hardly", "scarcely", "barely", "don't", "doesn't", "didn't", "isn't", "aren't", "wasn't", "weren't", "can't", "couldn't", "shouldn't", "wouldn't", "won't", "shan't"}
      if not sentence:
          return "Affirmative"
      words = set(sentence.lower().split())
      if words & negation_words:
          return "Negative"
      return "Affirmative"