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