proposition: Lemma subsetDP A B C : reflect (A \subset B /\ [disjoint A & C]) (A \subset B :\: C).
proof: Proof. by rewrite subsetD; apply: andP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)