input: Write a function to extract the verbs from a given sentence. The function should return a list of verbs. Use the NLTK library to tokenize the sentence and tag the parts of speech. If the sentence is empty, return an empty list.
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_vars("The dog is running fast.")==['is', 'running']
  assert extract_verbs("She sings beautifully.")==['sings']
  assert extract_verbs("")==[]
  assert extract_verbs("Reading is fun.")==['Reading']
output: def extract_verbs(sentence):
      if not sentence:
          return []
      from nltk import word_tokenize, pos_tag
      tokens = word_tokenize(sentence)
      tagged = pos_tag(tokens)
      verbs = [word for word, pos in tagged if pos.startswith('VB')]
      return verbs