proposition: Lemma fsetDDr A B C : A `\` (B `\` C) = (A `\` B) `|` (A `&` 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)