Lemma firstelement_le {n : nat} : ∏ i : ⟦S n⟧, @firstelement n ≤ i. Proof. intros. apply idpath. Defined.
Require Import UniMath.MoreFoundations.Tactics. Require Import UniMath.MoreFoundations.DecidablePropositions. Require Import UniMath.MoreFoundations.NegativePropositions.