Q: include-ing signatures of nested structures In module-a.sig, I have:
signature FOO = sig
  structure Bar: sig
    end
end

which corresponds to module-a.sml, which looks like:
structure Foo: FOO = struct
  structure Bar = struct 
    end
  end 

In module-b.sml, I need to write something that extends all of the types in Foo. It looks like this:
structure Baz: BAZ = struct 
  structure Bat = struct 
    open Foo.Bar
    end
  end

How can I write a signature for Baz that shows that it's got all of FOO.Bar inside of it? If Bar were top level, with a signature named BAR, I would write:
signature BAZ = sig
  structure Bat: sig
    include BAR
  end
end

but I can't do that here, because there is no top-level signature named BAR. I tried:
signature BAZ = sig
  structure Bat: sig
    include FOO.Bar
  end
end

but that caused a syntax error. I also tried:
signature BAZ = sig
  include FOO
  structure Bat: sig
    include Bar
  end
end

but that gave me "Error: unbound signature: Bar". 
Can this be done? Can I avoid writing out a top-level signature for Bar somewhere?
