proposition: Lemma length_fill {A: UU} (a: A) (n: nat): length (fill a n) = n.
proof: Proof. apply idpath. Defined.
imports: UniMath.Foundations.NaturalNumbers UniMath.Combinatorics.Vectors UniMath.Combinatorics.Lists UniMath.Combinatorics.DecSet UniMath.Combinatorics.Maybe