public override Tuple<AST.Env, AST.ExprType> GetExprTypeEnv(AST.Env env, Boolean is_const, Boolean is_volatile) {
            if (enums == null) {
                // if there is no content in this enum type, we must find it's definition in the environment
                Option<AST.Env.Entry> entry_opt = env.Find($"enum {name}");
                if (entry_opt.IsNone || entry_opt.Value.kind != AST.Env.EntryKind.TYPEDEF) {
                    throw new InvalidOperationException($"Type 'enum {name}' has not been defined.");
                }
            } else {
                // so there are something in this enum type, we need to put this type into the environment
                Int32 idx = 0;
                foreach (Enumerator elem in enums) {
                    Tuple<AST.Env, String, Int32> r_enum = elem.GetEnumerator(env, idx);
                    env = r_enum.Item1;
                    String name = r_enum.Item2;
                    idx = r_enum.Item3;
                    env = env.PushEnum(name, new AST.TLong(), idx);
                    idx++;
                }
                env = env.PushEntry(AST.Env.EntryKind.TYPEDEF, "enum " + name, new AST.TLong());
            }

            return new Tuple<AST.Env, AST.ExprType>(env, new AST.TLong(is_const, is_volatile));
        }