[Ur] Folders as implicit constructor arguments?

[Adam Chlipala]

Robin Green wrote:
> Since folder is declared (in the standard library) as a constructor
> synonym, and Ur/Web can infer folders, shouldn't folders be considered
> "implicit constructor arguments" for the purpose of @ and @@? The
> reference manual doesn't mention folders when it talks about @ and @@.
>    

I've updated the manual to reflect that folders are implicit arguments 
by default.

P.S.: Small nitpick: I'm not entirely comfortable with saying that 
[folder] is declared as a constructor synonym, since 'top.urs' hides 
this fact.