I just got it! (invariant/const)

[Robert Fraser]

Lionello Lunesu wrote:
> One thing that I like is that suddenly C++ "const &" construction can be 
> standardized and made safe at the same time! Because of invariant, "in" 
> can safely pass the arguments by ref, instead of by value.

And the compiler can make the decision for you as to which would be more 
efficient on your architecture. Happiness all around.