Method lift

src/examples/rpp/doxygen/readme.cpp:40–46

    // 3: implement core logic of operator: accept downstream observer (of result_type) and convert it to upstream observer (of T).
    template<typename Upstream, rpp::constraint::observer Observer>
    auto lift(Observer&& observer) const
    {
        const auto dynamic_observer = std::forward<Observer>(observer).as_dynamic();
        return rpp::make_lambda_observer<Upstream>([dynamic_observer, fn = fn](const auto& v) { dynamic_observer.on_next(fn(v)); },
                                                   [dynamic_observer](const std::exception_ptr& err) { dynamic_observer.on_error(err); },
                                                   [dynamic_observer]() { dynamic_observer.on_completed(); });
    }
};

template<typename Fn>