proposition: Lemma is_part_behead sh : is_part sh -> is_part (behead sh).
proof: Proof. case: sh => [//| l0 sh] /=; exact: is_part_consK. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(div ssralg ssrint ssrnum binomial) tools combclass sorted ordtype