open Jog open Zone;;
let top = [[0; 1; 2; 3; 4]; [0; 1; 2; 4; 5]; [0; 1; 2; 5; 6]; [0; 1; 2; 6; 3]]
and ini = ref (fun _ -> raise (OffTrack 3)) in
let bndry = Glue.morphgen (nz (fun i j ->
     if i=j then 0. else
     if i>2 && j>2 then 4. /. 3. else 1.)) top
    [([0; 1; 2; 3], ((fun  x -> x),
      (fun t -> ini := t.curl; {t with curl = (fun (k, ball) -> ball)})))] in
    (bndry, !ini (3, Linear.iden 4))

(* should yield (_, iden 4) *);;