Canonical ssrnat.nat_eqType.

Lemma interp_AppExt_set_nth_not_const :
forall k f le l n x,
interp (set_nth 0 l n x) (AppExt k f le) =
apply k f (foldr (fun v acc i => if ssrnat.eqn i v then interp (set_nth 0 l n x) (nth (Cst 0) le v) else acc i)
(nth 0 (map (interp l) le)) (index_not_const le n)).