input: Write a function to generate a Python program that takes a sentence as input and prints out only the verbs.
  
  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 eating a delicious apple.")==["is", "eating"]
  assert extract_verbs("Walking in the park is enjoyable.")==["Walking", "is"]
  assert extract_verbs("Running makes him happy.")==["Running", "makes"]
  assert extract_verbs("")==[]
output: 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