[Ur] Polymorphic variants and JSON

[Edward Z. Yang]

Excerpts from Edward Z. Yang's message of Sat Apr 14 19:29:32 -0400 2012:
> This particular permutation gives me:
> 
>     /home/ezyang/Dev/logitext/meta/json.ur:300:55-300:69: Can't resolve type class instance
>     Class constraint:  Top.folder[[Type]] rest
> 
> and I don't know how to spell the implicit arguments correctly or what
> folder I should be giving it (it's /almost/ 'fl' except that the
> entries in front need to be lopped off).  Is there a version of fold which
> also gives me folders?

Here's an idea: fold once to get a record of folders, and fold again
with that. But maybe the unification won't be clever enough.

Edward