structure descriptions and where structure?

I was thinking briefly about how "where structure" might be implemented 
today, when I discovered while looking at _The Definition of Standard ML 
(Revised)_ that I seem to have been making use of a feature in SML/NJ 
that is not part of the definition, namely, I have been using the 
following quite a bit recently


signature FOO = sig

   (* ... *)

   structure Baz : BAZ = ABaz

   (* then more stuff *)

end


just looking at the definition and the HaMLet implementation indicate 
that only


signature FOO = sig

   (* ... *)

   structure Baz : BAZ

   (* then more stuff *)

end


is allowed.  This leads me to ask (a) is there a paper somewhere that 
describes SML/NJ's implementation of this extension, (b) do any other 
SML implementations have this extension, and (c) is there a reason that 
SML/NJ does not implement "where structure" already?  As is, this is a 
very useful extension, but it would be even better in conjunction with 
"where structure".  Furthermore, while studying the problem I noticed 
that in the definition


signature FOO = sig

   type t = int

end


is not strictly allowed but is a derived form for


signature FOO = sig

   include
     sig type t end
     where type t = int

end


Which lead me to wonder why might


signature FOO = sig

   structure Baz : BAZ = ABaz

end


not be a derived form of


signature FOO = sig

   include
   sig structure Baz : BAZ end
   where structure Baz = ABaz

end


in SML/NJ.  In any event, perhaps this will lead some discussion that 
can evolve into a proposal for "where structure" on the Successor ML wiki.



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