proposition: Lemma right_sizes_cat_left t1 t2 : right_sizes (cat_left t1 t2) = right_sizes t2 ++ right_sizes t1.
proof: Proof. rewrite /cat_left right_sizes_from_left rev_cat map_cat flatten_cat. by rewrite -!right_sizes_from_left !left_branchK. Qed.
imports: HB(structures) mathcomp(all_ssreflect) tools combclass ordtype