expr : expr[ var_name ] '+' ident[ other_var_name ] ( some_action(var_name, other_var_name)) | ident[ third_var ] (some_other_action(third_var))

std::wstring ShiftIdent() {
    auto ident = current_token->codepoints; // for example
    current_token++;
    return ident;
}

void ShiftPlus() {
    current_token++;
}

T ShiftExpr() {
    T t;
    switch(current_token->type) {
    case ident:
        auto internal_var = ShiftIdent();
        while(true) {
            switch(current_token->type) {
            case '+':
                ShiftPlus();
                switch(current_token->type) {
                case ident:
                    auto&& other_var_name = ShiftIdent();
                    {
                        auto&& other_var_name = internal_var;
                        auto&& var_name = t;
                        t = some_action(var_name, other_var_name));
                    };
                    break;
                default:
                    error();
                }
                break;
            default:
                {
                    auto&& third_var = internal_var;
                    return some_other_action(third_var);
                }
            };
        }
        break;
    default:
        error();
    }
}