proposition: Corollary length_eq0 s : length s = 0 -> s = 1.
proof: Proof. rewrite -size_canword -{2}(canwordP s) => /eqP/nilP ->. exact: big_nil. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(fingroup perm morphism presentation) mathcomp(ssralg poly ssrint) permcomp tools permuted combclass congr