[Ur] blog post on fold function

[Adam Chlipala]

hao deng wrote:
>      I write a little document about fold function on following blog.
> http://haodeng.blogspot.com/
>    

Looks mostly accurate.  A few notes:

> val fold : K - a kind, one of Type, Name, or Unit.

The language of kinds is infinite, containing more than these three 
choices.  Its grammar appears in the manual.

> -> folder ra - folder that will derived by the compiler

Actually, in some cases it is useful to build [folder]s manually, using 
the combinators from [Top.Folder].

P.S.: I notice that your blog post contains no context information or 
links to Ur/Web info, so you may have readers who wonder what the heck 
you're talking about. ;)