> Many thanks! I just had a glance at it, but it seems to be just how one
> would have to approach such a problem. (The issue with hash-based
> approaches to find previously visited substructures is that during
> traversal, a GC may occur. Now I just assume that this may involve
> relocation and heap compaction in OCaml. The problem then is that
> OCaml does not properly support what would be known as eq hash tables
> in Lisp.)

As long as the data structure supports the polymorphic hash function, it 
should work to simply use a regular hash table with the polymorphic hash 
function and physical equality, as in:

module S = Hashtbl.Make(struct
   type t = Obj.t
   let hash = Hashtbl.hash
   let equal = (==)
end);;


(Of course, this might be quite slow.)

-- Alain