docs: public void createLabelFeatures(Collector fv, DependencyInstance inst,
                                      int[] heads, int[] types, int mod, int order) {
          int head = heads[mod];
          int type = types[mod];
          if (order != 2)
              createLabeledArcFeatures(fv, inst, head, mod, type);
  
          int gp = heads[head];
          int ptype = types[head];
          if (order != 1 && gp != -1) {
              createLabeledGPCFeatureVector(fv, inst, gp, head, mod, type, ptype);
          }
      }