Homework help, guys!

[hunt]

Hello, guys. It is weird to ask about my homework here, but I 
haven't gotten a proper answer yet. It's about D feature...maybe..

http://dpaste.dzfl.pl/905d6ad7

Question is that "Investigate how D allows you to defend your 
definition of Map so that it may be used only with a type of key 
that is comparable, and place this defense in your solution."

Please leave your comments here. Thank you so much.