code: def _get_terminals_and_nonterminals(sentence_graph):
      terminals = set()
      nonterminals = set()
      for node_id in sentence_graph.nodes_iter():
          if sentence_graph.out_degree(node_id) > 0:
              nonterminals.add(node_id)
          else:
              terminals.add(node_id)
      return sorted(list(terminals), key=natural_sort_key), \
          sorted(list(nonterminals), key=natural_sort_key)