proposition: Lemma fsetDDl A B C : (A `\` B) `\` C = A `\` (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)