private RegularExpresionNode F() throws ParserException
    {
        if (currentSymbol == '(')
        {
            currentSymbol = getNextSymbol();
            RegularExpresionNode subExpresionNode = ER();
            if (currentSymbol != ')')
                throw new ParserException("Was expecting a )");
            currentSymbol = getNextSymbol();
            return subExpresionNode;
        }
        else
            return Symbol();
    }