I'd like to do something like the following:

type snode = Empty | Snode of CharSet.t
and
module CharSet = Set.Make(struct type t = char * snode
		            let compare (x,_) (y,_) = Pervasives.compare x y
			  end)

That is define an inter-dependent typing relationship between two
modules (although there is only one module here, the other can be
viewed as the `null' module). Is this possible?  If not is there
another way to achieve the effect?

Cheers,
Julian.