proposition: Lemma fsetIAC A B C : A `&` B `&` C = A `&` C `&` B.
proof: Proof. by rewrite -!fsetIA (fsetIC B). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)