proposition: Lemma fsetIA A B C : A `&` (B `&` C) = A `&` B `&` C.
proof: Proof. by apply/fsetP=> x; rewrite !inE andbA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)