proposition: Lemma dmO s t : mprv (~~: (s :\/: t) <--> (~~: s) :/\: (~~: t)).
proof: Proof. rewrite /Or /And. rule axAI; by rewrite -> axDNE. Qed.
imports: mathcomp(all_ssreflect) CompDecModal.libs(edone base fset induced_sym) Setoid Morphisms Relation_Definitions