From mathcomp.ssreflect Require Import ssreflect ssrbool ssrnat ssrfun eqtype seq fintype finfun.
Set Implicit Arguments.
(* given n <= m, returns the list [n-1,...,0] with proofs of < m *)
Program Fixpoint enum_from n m (pr : le n m) : list (ordinal m) :=
match n with
O => nil
| S n => (@Ordinal m n ltac:(rewrite <-Heq_n in *; apply (introT leP pr)))
:: @enum_from n m ltac:(rewrite <-Heq_n in *; apply Le.le_Sn_le, pr)
end.
Definition enum n := Coq.Lists.List.rev (@enum_from n n (Le.le_refl n)).
Fixpoint countdown (n: nat) := match n with O => nil | S n' => n' :: countdown n' end.

Lemma countdown_rev_iota:  forall n, countdown n = List.rev (iota 0 n).