Purity in Eiffel

[Max Klyga]

Found a blog post about purity and contracts relations in Eiffel 
language. Thought that would be interesting for D comminity.
This post also mentions term "weak purity" that differs from what it 
means in D.

http://bertrandmeyer.com/2011/07/04/if-im-not-pure-at-least-my-functions-are/