Method evaluate

test/github_issues.cpp:649–652

    {
        explicit parenthesized_evaluatable(std::unique_ptr<evaluatable>&& e) : evaluatable_(std::move(e)) {}
        double evaluate() override
        {
            return evaluatable_->evaluate();
        }
        std::unique_ptr<evaluatable> evaluatable_;
    };
    constexpr bp::rule<struct parenthesized_tag, std::unique_ptr<evaluatable>> parenthesized_parser = "parenthesized_parser";