Section Test.
Variable (a b x y z : X).
(* The following goals should hold by computation *)
Goal fill [|Fin0; Fin1|] [|x;y;z|] [|a;b|] = [|a;b;z|].
Definition fill_default I (def : X) V :=
fill I (Vector.const def n) V.

Corollary fill_default_not_index I V def i :
not_index I i ->
(fill_default I def V)[@i] = def.