input: Write a function to extract verbs from a given sentence using the NLTK library. The function should take a sentence as input and return a list of verbs. If the input sentence is empty, the function should return an empty list.
  import nltk
  
  def extract_verbs(sentence):
      if not sentence:
          return []
      tokens = nltk.word_tokenize(sentence)
      tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tags if pos.startswith('V')]
      return verbs
  
  sentence = "The cat jumped over the moon."
  assert extract_verbs(sentence) == ['jumped', 'over']
  
  sentence = "She is running very fast."
  assert extract_verbs(sentence) == ['is', 'running', 'fast']
  
  sentence = ""
  assert extract_verbs(sentence) == []
  
  sentence = "He was sleeping and dreaming"
  assert extract_verbs(sentence) == ['was', 'sleeping', 'dreaming']
  
  sentence = "The book on the table"
  assert extract_verbs(sentence) == []
output: def extract_verbs(sentence):
      if not sentence:
          return []
      tokens = nltk.word_tokenize(sentence)
      tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tags if pos.startswith('V')]
      return verbs