type xpair = XPair : (_ * _) ty' -> xpair

let tpair : type a b. a ty' -> b ty' -> (a * b) ty' =
 fun a b ->
  let key = (Xid a.id, Xid b.id) in
  try
    let (XPair r) = Hashtbl.find tpair_table key in
    let TPair (a', b') = r.value in
    match (eq_id a.id a'.id, eq_id b.id b'.id) with
    | Some Eq, Some Eq ->
        r
    | _ ->
        assert false
  with Not_found ->
    let r = {id= gen (); value= TPair (a, b)} in
    Hashtbl.add tpair_table key (XPair r) ;
    r

We can remove an `assert_false` by putting `XPair` in the hashtbl: ```ocaml type xpair = XPair : (_ * _) ty' -> xpair let tpair : type a b. a ty' -> b ty' -> (a * b) ty' = fun a b -> let key = (Xid a.id, Xid b.id) in try let (XPair r) = Hashtbl.find tpair_table key in let TPair (a', b') = r.value in match (eq_id a.id a'.id, eq_id b.id b'.id) with | Some Eq, Some Eq -> r | _ -> assert false with Not_found -> let r = {id= gen (); value= TPair (a, b)} in Hashtbl.add tpair_table key (XPair r) ; r ``` The other one is indeed based on the fact that passed parameters are themselves hashcons'ed and I don't think it's possible to enforce this in the type system.