gistfile1.sml

   val state : unit cont ref =
       ref (callcc (fn escape : unit cont cont =>
                       (callcc (fn k => throw escape k);
                        raise AmbFail)))

   fun amb [] = throw (!state) ()
     | amb (x::xs) =
       let val old = !state
       in callcc (fn escape =>
                     (callcc (fn k => (state := k; throw escape x));
                      state := old;
                      amb xs))
       end

   (* Collect all possible results of the nondeterministic
    * computation t (that is, t might use amb) *)
   fun collect t =
       let val old = !state
           val results = ref []
           val () = callcc
                    (fn finished =>
                        (state := finished;
                         results := t () :: !results;
                         amb [] (* backtrack *)))
           val () = state := old
       in rev (!results) end