proposition: Lemma lt_sorted_mask m s : sorted <%O s -> sorted <%O (mask m s).
proof: Proof. exact/sorted_mask/lt_trans. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)