@Override
    public String toString() {
        String as = a.toString();
        String bs = b.toString();
        if(as.startsWith("(") && as.endsWith(")")) as = as .substring(1,as.length()-1);
        if(bs.startsWith("(") && as.endsWith(")")) bs = bs .substring(1,bs.length()-1);
        return "(" + as + "|" + bs + ")";
    }