(** sorted *)
Fixpoint sorted {T : Type} (Ord : T -> T -> Prop) (s : seq T) :=
match s with
| [::] | [:: _] => True
| h0 :: (h1 :: t1) as t0 => Ord h0 h1 /\ sorted Ord t0
end.

Lemma sorted_compat (s : seq R) :
sorted Rle s <-> ordered_Rlist (seq2Rlist s).