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