proposition: Lemma is_part_decr_nth_part (p : intpart) i : is_part (if is_rem_corner p i then decr_nth p i else p).
proof: Proof. case: (boolP (is_rem_corner p i)). - by apply is_part_decr_nth; apply: intpartP. - by move=> _; apply: intpartP. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(div ssralg ssrint ssrnum binomial) tools combclass sorted ordtype