[MLton] signature language lacking

[Florian Weimer]

* Henry Cejtin:

> Am I missing something?

Not sure, what about this?

	local
	    type at = S.at
	    type bt = S.bt
	in
	structure S: sig

	   structure A: sig
	      type t = at
	      val f: at -> bt
	   end

	   structure B: sig
	      type t = bt
	      val f: bt -> at
	   end
	end = S
	end

Something like this is already necessary when you want to add a value to
a structure without specifying its type, and without cluttering the
namespace.