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