proposition: Lemma axAUAEr s t u : mprv (AU (s :/\: u) (t :/\: u) ---> u).
proof: Proof. apply: rAU_ind => //. rewrite -> axAEr. by K. Qed.
imports: mathcomp(all_ssreflect) CompDecModal.libs(edone base fset induced_sym) Setoid Morphisms Relation_Definitions