proposition: Lemma dmER s t : mprv ((~~: ER s t) <--> AU (~~: s) (~~: t)).
proof: Proof. rewrite /ER. rule axAI; by do ! rewrite -> axDNE. Qed.
imports: mathcomp(all_ssreflect) CompDecModal.libs(edone base fset induced_sym) Setoid Morphisms Relation_Definitions