proposition: Lemma fsetSD C A B : A `<=` B -> A `\` C `<=` B `\` C.
proof: Proof. move=> sAB; apply/fsubsetP=> x; rewrite !inE. by case: (x \in C) => //; exact: (fsubsetP sAB). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)