proposition: Lemma iter_up_ren_lt {n ξ x} : x < n -> Nat.iter n up_ren ξ x = x.
proof: Proof. elim: n x ξ; first by lia. move=> n IH [|x] ξ /= ?; first done. by rewrite /funcomp IH; first by lia. Qed.
imports: List Lia Undecidability.SystemF.SysF Undecidability.SystemF.Autosubst.syntax Undecidability.SystemF.Autosubst.unscoped Undecidability.SystemF.Util.Facts ssreflect ssrbool ssrfun