I give away five vouchers for my book "C++20". In return, I have a question about the Big Four: concepts, ranges, coroutines, and modules. Which feature excites your most and why?

Here is a short presentation of my book.
My book C++20 is both: a tutorial and a reference to the C++20 standard. It teaches you C++20 and provides you with the details of this new thrilling C++ standard. The thrilling factor is mainly due to the big four of C++20.
- Concepts change the way we think and program templates. They are semantic categories for the template parameters. They enable you to express your intention directly in the type system. If something goes wrong, you get a clear error message.
- The new ranges library enables it to perform algorithms directly on the container, compose the algorithm with the pipe symbol, and apply them onto infinite data streams.
- Thanks to coroutines asynchronous programming in C++ becomes mainstream. Coroutines are the base for cooperative tasks, event loops, infinite data streams, or pipelines.
- Modules overcome the restrictions of header files. They promise a lot. For example, the separation of header and source files becomes as obsolete as the preprocessor. In the end, we have faster built time and an easier way to build packages.
If you want more information about the book including the TOC, you can have it here:
https://leanpub.com/c20. The book includes more than 150 code examples.
How can you win a copy of the book: This is simple. I want to know, which feature of the Big Four concepts, ranges, coroutines, and modules excites you most and why? The five best answers will get the coupon.
What are the rules: Send your answer in English until Monday, the 15th of February to This email address is being protected from spambots. You need JavaScript enabled to view it..
Please share this post. I'm curious about your answers.
Modernes C++,
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Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, and Richard Sargeant.
My special thanks to Embarcadero 
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