code: protected static String getPayload(SAXSymbol symbol) {
      if (symbol.isGuard()) {
        return "guard of the rule " + ((SAXGuard) symbol).r.ruleIndex;
      }
      else if (symbol.isNonTerminal()) {
        return "nonterminal " + ((SAXNonTerminal) symbol).value;
      }
      return "symbol " + symbol.value;
    }
docs: This routine is used for the debugging.
  
  @param symbol the symbol we looking into.
  @return symbol's payload.