fact: Definition ribbon_on start stop inner outer := [/\ forall i, i > stop -> nth 0 outer i = nth 0 inner i, forall i, start <= i < stop -> nth 0 outer i.+1 = (nth 0 inner i).+1, nth 0 inner start < nth 0 outer start & forall i, i < start -> nth 0 outer i = nth 0 inner i].
imports: HB(structures) mathcomp(all_ssreflect) tools sorted partition