diff --git a/playground/jfla.ml b/playground/jfla.ml
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+(* I. Introduction *)
+
+type nat = Zero | Succ of nat
+
+type 'a stream = ('a cell) Lazy.t and 'a cell = Cell of 'a * 'a stream
+
+let rec from : nat -> nat stream = fun n -> lazy (Cell (n, from (Succ n)))
+
+let naturals = from Zero
+
+let rec nth : nat -> 'a stream -> 'a = fun n s ->
+  let Cell (hd, tl) = Lazy.force s in
+  match n with
+  | Zero -> hd
+  | Succ m -> nth m tl
+
+type 'a stream = {
+  Head : 'a;
+  Tail : 'a stream;
+}
+
+let corec from : nat -> nat stream with
+  | (.. n)#Head -> Zero
+  | (.. n)#Tail -> from (Succ n)
+
+let rec nth n s = match n with
+  | Zero -> s#Head
+  | Succ m -> nth m s#Tail
+
+
+let corec map2 : type a b c. (a -> b -> c) -> a stream -> b stream -> c stream with
+  | (.. f s1 s2)#Head -> f s1#Head s2#Head
+  | (.. f s1 s2)#Tail -> map2 f s1#Tail s2#Tail
+
+(* III. Constructions avancées du filtrage par comotifs *)
+
+let corec fib : int stream with
+   | ..#Head -> 0
+   | ..#Tail : int stream with
+   | ..#Tail#Head -> 1
+   | ..#Tail#Tail -> map2 ( + ) fib fib#Tail
+
+let corec lazy fib : int stream with
+   | ..#Head -> 0
+   | ..#Tail : int stream with
+   | ..#Tail#Head -> 1
+   | ..#Tail#Tail -> map2 ( + ) fib fib#Tail
+
+let corec cycle : nat -> nat stream with
+   | (.. n)#Head -> n
+   | (.. Zero)#Tail -> cycle (Succ (Succ (Succ Zero)))
+   | (.. (Succ m))#Tail -> cycle m
+
+type zero = Z and 'a succ = S
+
+type 'a fuel =
+  | Dry : zero fuel
+  | More :  'a fuel -> ( 'a succ) fuel
+
+type ('a,'b) bounded_iterator = {
+  GetVal :  'a;
+  Next : ('a,'b) bounded_iterator <- ('a, 'b succ) bounded_iterator;
+}
+
+module type Seq = sig
+  type 'a t
+  val head : 'a t -> 'a
+  val tail : 'a t -> 'a t
+end
+
+module MkIterator (S : Seq) = struct
+  let corec wrap : type a b.a S.t -> b fuel -> (a, b) bounded_iterator with
+     | (.. l n)#GetVal -> S.head l
+     | (.. l (More n))#Next -> wrap (S.tail l) n
+end
+
+(* IV. Destructeurs de premier ordre et première classe *)
+
+type _loc = {name: string; x : int; y : int}
+
+let select_name  lc = lc.name   and update_name  s lc = {lc with name = s}
+let select_x     lc = lc.x      and update_x     b lc = {lc with x = b}
+let select_y     lc = lc.y      and update_y     n lc = {lc with y = n}
+
+type loc = {Name : string; X : int; Y: int}
+
+let select (d : 'a loc_obs) (Loc {dispatch} : loc) : 'a = dispatch d
+
+type (_,_) eq = Eq : ('a,'a) eq
+
+let eq_loc : type a b. a loc_obs * b loc_obs -> ((a,b) eq) option = function
+  | (Name, Name) -> Some Eq
+  | (X, X) -> Some Eq
+  | (Y, Y) -> Some Eq
+  | _ -> None
+
+let update (type a) (d1 : a loc_obs) (x : a) (Loc {dispatch}) =
+   let dispatch : type o. o loc_obs -> o = fun d2 -> match eq_loc (d1,d2) with
+     | Some Eq -> x
+     | _ -> dispatch d2
+   in Loc {dispatch}