(*s The order [Dle] extended to $D_{\infty}$ *)
Definition Ddle (dd dd' : option D) :=
match dd, dd' with
| _, None => true
| None, _ => false
| Some d, Some d' => Dle d d'
end.

Lemma Ddmin_le_2 : forall dd dd' : option D, Ddle (Ddmin dd dd') dd' = true.