proposition: Lemma subEproper A B : A \subset B = (A == B) || (A \proper B).
proof: Proof. by rewrite eqEsubset -andb_orr orbN andbT. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)