Doc writing for unpure-pure-containers

[address@hidden]

Hey all,

I've answered a couple questions recently where unpure-pure-containers have 
come in handy and think it'd be beneficial to have some text in the notation 
manual about them.  However, understanding pure properties is scary for 
developers, so I'd imagine that it'd be downright nightmarish for users.  James 
- would you be interested in helping me out with this? I think that you're good 
at formulating things in a way that people understand.

Cheers,
MS