Definition Pred : Cobham :=
Rec Zero (Proj 2 0) (Proj 2 0) (Proj 1 0).
(* Eval vm_compute in (arity pred'). *)
Definition move_arg (n i j:nat) (e:Cobham) : Cobham :=
Comp n e (move_forward i j
(map (Proj n) (seq 0 n)) (Proj n 0)).

Lemma move_arg_inf n i j e :
i+j < n -> arity e = ok_arity n ->
arity (move_arg n i j e) = ok_arity n.