proposition: Lemma reduced_catl u v : u ++ v \is reduced -> u \is reduced.
proof: Proof. Proof using. move/reduced_rev; rewrite rev_cat => /reduced_catr. by rewrite -reduced_revE. Qed.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(fingroup perm morphism presentation) mathcomp(ssralg poly ssrint) permcomp tools permuted combclass congr