private static String getRuleName(RuleType combinator) {
      // fa ba fc bx gfc gbx  conj
      switch (combinator) 
      {
      case FA : return "fa";
      case BA : return "ba";
      case FC : return "fc";
      case BX : return "bx";
      case CONJ : return "conj";
      case RP : return "rp";
      case LP : return "lp";
      case GFC : return "gfc";
      case GBX : return "gbx";
      }

      throw new RuntimeException("Unknown rule type: " + combinator);
    }