mutually recursive classes (yes - again)

[briand@aracnet.com]

I've read the other threads on this common subject and I am not
getting understanding.

So I present the following very simple example with the hopes that
some kind person will help.

The definition is very simple making use of the "and" keyword 

class type a1 = 
object
  val a:a1
  val b:a2
end
and a2
object
  val c:a2
  val d:a1
end
;;

It's the implementation which gives me trouble.  And yes there is an
example in the manual (3.15) and it just doesn't go quite far
enough...

now I have to define the clases

class a1 (x:a1) (y:a2) =
object
  val a = x
  val b = y
end
;;

class a2 (x:a2) (y:a1) =
object
  val c = x
  val d = y
end
;;

So now 

let z1 = new a1 ?? ?? ;;
let z2 = new a2 ?? ?? ;;

Obviously I need to "ground" the recursion somehow, but without some
sort of "Null" or "Void" value to use to instantiate the a1 field of
a1 it's not obvious to me how to do it.

I'm sure it will be embarassingly simple ...

I have about 7 more classes in this recursion-fest and a nice orderly
way to handle it would be VERY helpful.

TIA.

Brian