proposition: Lemma msetBSS A B C D : A `<=` C -> D `<=` B -> A `\` B `<=` C `\` D.
proof: Proof. by move=> /(msetSB B) /msubset_trans sAC /(msetBS C) /sAC. Qed.
imports: mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop tuple) finmap