[Ur] Record subtype constraint

[Adam Chlipala]

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].