I noticed this on http://wiki.darcs.net/HowToHelp

> The new FL and RL types are great for safety, but they don't get deforested. >
Implement a RULES system (maybe Don's streams?) to allow deforestation/fusion.

FL and RL are just lists that enforce the order of elements with the type
system.  If you cons an p y z to an FL p x y, you get back an FL p x z.  If you
reverse a FL p x y you get back an RL p y x.

Sounds like something a non-Darcs-savvy Haskell hacker who work on.  I'm marking
this as ProbablyEasy because it sounds like it can be isolated to one module
(Darcs.Ordered -- which I still think ought to live on as its own package)