Lemma stnsum_step {n : nat} (f: ⟦S n⟧ -> nat) : stnsum f = stnsum (f ∘ (dni lastelement)) + f lastelement. Proof. intros. apply idpath. Defined.
Require Import UniMath.MoreFoundations.Tactics. Require Import UniMath.MoreFoundations.DecidablePropositions. Require Import UniMath.MoreFoundations.NegativePropositions.