proposition: Lemma ribbon_mindropeq inner outer : is_part inner -> is_part outer -> ribbon inner outer -> let min := mindropeq inner outer in let height := ribbon_height inner outer in ribbon_on (min - height) min.-1 inner outer.
proof: Proof. move=> partinn partout /(ribbonP partinn partout) [start][stop] Hrib /=. rewrite -(ribbon_on_height partinn partout Hrib). rewrite (ribbon_on_mindropeq partinn partout Hrib) /=. by rewrite subSS subKn // (ribbon_on_start_stop Hrib). Qed.
imports: HB(structures) mathcomp(all_ssreflect) tools sorted partition