[Ur] Record subtype constraint

[Karn Kallio]

> Karn Kallio wrote:
> > However, this does not compile:
> > 
> > [codesnip]
> > 
> > giving error:
> > 
> > f.ur:11:4-11:6: Expression is not a constructor function
> > Expression:  fl
> > 
> >        Type:  Top.folder[[Type]] super
> 
> This makes sense, since you are trying to apply [fl] as a constructor
> function, but it isn't one, it's a [folder].  [folder] is an abstract
> type family.  You should instead be calling [Top.fold], which can be
> abbreviated to just [fold].

Aha!  Thanks!  In case anyone else wonders how to express record subtype 
constraints I posted a page on the wiki : 
http://www.impredicative.com/wiki/index.php/Expressing_subrecord_constraints