private void addDefaultTVarAndEVarIfNotExist(SupportCodeGenerator scg) {
        // add default type and environemtn variable
        if (tvarList.isEmpty()) {
            tvarList.add("__t");
            if(scg.T.startSymbol==null) {
                scg.defTypeVar("__tS", "__t");
            } else {
                scg.defTypeVar(scg.T.startSymbol, "__t");
            }
        }
        if (evarList.isEmpty()) {
            evarList.add("__E");
            if(scg.E.startSymbol == null) {
                scg.defEnvVar("env", "__E");
            } else {
                scg.defEnvVar("__ES", "__E");
            }
        }
    }