proposition: Lemma outer_shapeK inner diff : size inner <= size diff -> (outer_shape inner diff) / inner = diff.
proof: Proof. rewrite /outer_shape. elim: inner diff => [| inn0 inn IHinn] /= diff. rewrite subn0 => _; elim: diff => //= d0 diff ->. by rewrite add0n. case: diff => //= t0 t; rewrite ltnS subSS => /IHinn /= ->. by rewrite addKn. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(div ssralg ssrint ssrnum binomial) tools combclass sorted ordtype