proposition: Lemma rEXn s t : mprv (s ---> t) -> mprv (EX s ---> EX t).
proof: Proof. rewrite /EX => H. by rewrite -> H. Qed.
imports: mathcomp(all_ssreflect) CompDecModal.libs(edone base fset induced_sym) Setoid Morphisms Relation_Definitions