recursive polymorphic variants?

[nr@eecs.harvard.edu (Norman Ramsey)]

I'm trying to write a small, extensible interpreter, and I'd like to
use polymorphic variants as the extension mechanism.  But I'm getting
stuck on very simple things.  For example, I would like the value type
to include a few simple values, but I would also like it to be
extensible, thus:

  type value = [ `Nil
               | `Number   of float
               | `String   of string
               | `Function of [>value] list -> [>value]
               | `Table    of ([>value], [>value]) Hashtbl.t
               ]

However, when I do this, the compiler complains that

  The type constructor value is not yet completely defined

Is there some way to define a recursive, *extensible* type using
polymorphic variants?  



Norman