proposition: Lemma msetSB C A B : A `<=` B -> A `\` C `<=` B `\` C.
proof: Proof. by move=> /msubsetP sAB; apply/msubsetP=> x; rewrite !msetE leq_sub2r. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap