Required Reading: "How Non-Member Functions Improve Encapsulation"

[Walter Bright]

for core D devs.

"How Non-Member Functions Improve Encapsulation" by Scott Meyers

http://www.drdobbs.com/cpp/how-non-member-functions-improve-encapsu/184401197

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