Part 2 here: http://ticki.github.io/blog/lambda_crabs_2/
Ticki does really great work in Rust. I think the only thing he's doing that I disagree with is his RFC for a keyword for dynamic dispatch (all we really need is to distinguish trait objects - dynamic dispatch is just the symptom).
Ordered sets also occur in denotational semantics http://homepage.cs.uiowa.edu/~slonnegr/plf/Book/Chapter10.pd...
http://www.cliffc.org/blog/2012/03/24/too-much-theory-part-3...