proposition: Lemma in_fsetI A B a : (a \in A `&` B) = (a \in A) && (a \in B).
proof: Proof. by rewrite in_fset. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)