def get_nouns_verbs(list_of_trees):
  """ Returns the noun and verb tokens in a sentence as tuples: (word, ['v'|'n']) """
  ALLOWABLE = ['NN', 'NNS', 'NNP', 'NNPS', 
               'VB', 'VBD', 'VBG', 'VBN', 'VBP', 'VBZ']
  sentence_tokens = []
  for source_tree in list_of_trees:
    source_poses = source_tree.pos()
    for source_pos in source_poses:
      if source_pos[1] in ALLOWABLE:
        sentence_tokens.append((source_pos[0], source_pos[1][0].lower()))
  return sentence_tokens