assert false (* algorithm failed *)
let rec set_arity arity = function
+(* FIXME because onlt variables should be in branches of matches, one day *)
| `Var(n,_) -> `Var(n,arity)
| `N _ | `Bottom | `Pacman as t -> t
| `Lam(false, t) -> `Lam(false, set_arity arity t)