proposition: Corollary lengthM s t : length (s * t) <= length s + length t.
proof: Proof. rewrite -(size_canword s) -(size_canword t) -size_cat. have -> : s * t = 's_[canword s ++ canword t] by rewrite big_cat /= !canwordP. exact: length_prods. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(fingroup perm morphism presentation) mathcomp(ssralg poly ssrint) permcomp tools permuted combclass congr