proposition: Lemma subsetD1P A B x : reflect (A \subset B /\ x \notin A) (A \subset B :\ x).
proof: Proof. by rewrite subsetD1; apply: andP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)