proposition: Lemma fsetDSS A B C D : A `<=` C -> D `<=` B -> A `\` B `<=` C `\` D.
proof: Proof. by move=> /(fsetSD B) /fsubset_trans sAC /(fsetDS C) /sAC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)