proposition: Lemma length_sub1R s (i : 'I_n) : i < n0 -> s^-1 i > s^-1 (inord i.+1) -> length s = (length (s * 's_i)).+1.
proof: Proof. Proof using. move=> Hi Hdesc. rewrite -lengthV -[length (s * _)]lengthV invMg eltrV. exact: length_sub1L. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(fingroup perm morphism presentation) mathcomp(ssralg poly ssrint) permcomp tools permuted combclass congr