Variant last_spec : seq T -> Type :=
  | LastNil        : last_spec [::]
  | LastRcons s x  : last_spec (rcons s x).Variant last_spec : seq T -> Type :=
  | LastNil        : last_spec [::]
  | LastRcons s x  : last_spec (rcons s x).Variant last_spec : seq T -> Type :=
  | LastNil        : last_spec [::]
  | LastRcons s x  : last_spec (rcons s x).

Lemma nth_default s n : size s <= n -> nth s n = x0.