input: Write a function to extract verbs from a given sentence using NLTK. The function should take a sentence as input and return a list of verbs. If the input sentence is empty, return an empty list.
  import nltk
  
  def extract_verbs(sentence):
      tokens = nltk.word_tokenize(sentence)
      tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tags if pos.startswith('V')]
      return verbs
  
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She is running in the park.")==['is', 'running']
  assert extract_verbs("He wrote a letter.")==['wrote']
  assert extract_verbs("")==[]
  assert extract_verbs("The sun sets in the west.")==['sets']
output: import nltk
  
  def extract_verbs(sentence):
      tokens = nltk.word_tokenize(sentence)
      tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tags if pos.startswith('V')]
      return verbs