proposition: Lemma length_add1R s (i : 'I_n) : i < n0 -> s^-1 i < s^-1 (inord i.+1) -> length (s * 's_i) = (length s).+1.
proof: Proof. Proof using. move=> Hi Hdesc. rewrite -lengthV -[length s]lengthV invMg eltrV. exact: length_add1L. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(fingroup perm morphism presentation) mathcomp(ssralg poly ssrint) permcomp tools permuted combclass congr