proposition: Lemma fsetUDl (A B C : {fset K}) : A `|` (B `\` C) = (A `|` B) `\` (C `\` A).
proof: Proof. by apply/fsetP=> x; rewrite !inE; do ?case: (_ \in _). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)