Required Reading: "Functional Programming in C++"

[Walter Bright]

for core D devs. Of course, this is much easier in D than in C++ because of D's 
const and pure attributes.

"Functional Programming in C++" by John Carmack

https://www.gamasutra.com/view/news/169296/Indepth_Functional_programming_in_C.php

I'm as guilty as anyone for not understanding or following these guidelines. I 
expect we can do much better.