code: def extract_sentences(nodes, token_node_indices):
      
      sents = []
      tokens = []
      for i, node in enumerate(nodes):
          if i in token_node_indices:
              if node.features['tiger.pos'] != '$.':
                  tokens.append(i)
              else:  # start a new sentence, if 'tiger.pos' is '$.'
                  tokens.append(i)
                  sents.append(tokens)
                  tokens = []
      return sents
docs: given a list of ``SaltNode``\s, returns a list of lists, where each list
      contains the indices of the nodes belonging to that sentence.