input: Write a function to generate a Python program that takes a sentence as input and prints out only the verbs using the Natural Language Toolkit (nltk). The function should return a list of verbs. If the input sentence is empty, 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
  
  assert extract_verbs("The cat jumped over the moon.")==["jumped"]
  assert extract_verbs("I am running and you are walking.")==["am", "running", "are", "walking"]
  assert extract_verbs("She eats an apple.")==["eats"]
  assert extract_verbs("")==[]
  assert extract_verbs("Playing is fun.")==["Playing", "is"]
output: 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