"freeze"

[Timothy John Hickey]

Does anyone know how hard it would be to add Colmerauer's "freeze"
predicate to gnuprolog? It is a built-in of the form freeze(X,G) which 
has
the behavior that when X becomes a non-variable it calls the goal G.
For finite-domain variable, a natural interpretation would be to have it
call G when X has been contracted to a single value.

Thanks for any pointers,
---Tim Hickey