Class addition_evaluatable

test/github_issues.cpp:721–735

* ADDITION EXPRESSION */

     *      ADDITION EXPRESSION
     */
    struct addition_evaluatable : evaluatable
    {
        addition_evaluatable(std::vector<std::unique_ptr<evaluatable>>&& e)
            : evaluatables_(std::move(e))
        {}
        double evaluate() override
        {
            double result = 0;
            for (const auto& e : evaluatables_) {
                result += e->evaluate();
            }
            return result;
        }
        std::vector<std::unique_ptr<evaluatable>> evaluatables_;
    };
    constexpr bp::rule<struct add_tag, std::unique_ptr<evaluatable>> add_parser = "add_parser";
    constexpr auto add_parser_action = [](auto& ctx) {
        std::unique_ptr<evaluatable>& val = _val(ctx);