Lemma flattenStep {X} (x:list X) (m : list(list X)) : flatten (x::m) = concatenate x (flatten m). Proof. unfold flatten. rewrite list_ind_compute_2. apply idpath. Defined.
Require Import UniMath.MoreFoundations.Tactics. Require Import UniMath.Combinatorics.StandardFiniteSets. Require Import UniMath.Combinatorics.Vectors.