proposition: Lemma diff_shape_eq s : s / s = nseq (size s) 0.
proof: Proof. by elim: s => [|s0 a IHs] //=; rewrite subnn IHs. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(div ssralg ssrint ssrnum binomial) tools combclass sorted ordtype