proposition: Lemma fsetUDr (A B C : {fset K}) : (A `\` B) `|` C = (A `|` C) `\` (B `\` C).
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)