input: Write a function to extract verbs from a given sentence. The function should return a list of verbs found in the sentence. You can assume the nltk library is available for tokenization and part-of-speech tagging.
  
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She is running swiftly.")==['is', 'running']
  assert extract_verbs("He eats an apple.")==['eats']
  assert extract_verbs("The sun sets in the west.")==['sets']
  assert extract_verbs("They were singing a song.")==['were', 'singing']
output: def extract_verbs(sentence):
      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