code: void get_features(final State s,
                        final List<Integer> cluster4,
                        final List<Integer> cluster6,
                        final List<Integer> cluster,
                        List<Integer> features)
      {
          Context ctx = new Context();
          get_context(s, ctx);
          get_basic_features(ctx, s.ref.forms, s.ref.postags, s.deprels, features);
          get_distance_features(ctx, features);
          get_valency_features(ctx, s.nr_left_children, s.nr_right_children, features);
          get_cluster_features(ctx, cluster4, cluster6, cluster, features);
      }
docs: 生成特征
  @param s 当前状态
  @param cluster4
  @param cluster6
  @param cluster
  @param features 输出特征