proposition: Lemma fsetDP 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 ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)