std.container.RedBlackTree versus C++ std::set

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Again: My intention was not const&.

And you're right, but there was so many discussions about const& 
(since dmd 2.057; also in the last few days) and as every 
discussion here: after page 2 is the topic changed.
I'm also very sure that neither Walter nor Andrei see a 
(important) reason for something similar as const& because they 
don't need it. And if you don't need something, the priority for 
such thing is very low.
So everything we can do (after that much requests and 
discussions) is to wait what and when they will decide something.
I count the versions.