code: private static Label combineLabel(FA fa, IntBitSet states) {
          int si;
          Label tmp;
          Label result;
  
          result = null;
          for (si = states.first(); si != -1; si = states.next(si)) {
              tmp = (Label) fa.get(si).getLabel();
              if (tmp != null) {
                  if (result == null) {
                      result = new Label();
                  }
                  result.symbols.addAll(tmp.symbols);
              }
          }
  
          return result;
      }
docs: might return null