proposition: Lemma iter_up_ren_ge {n ξ x} : n <= x -> Nat.iter n up_ren ξ x = n + ξ (x-n).
proof: Proof. move=> ?. have {1}->: x = n + (x - n) by lia. by rewrite iter_up_ren. Qed.
imports: List Lia Undecidability.SystemF.SysF Undecidability.SystemF.Autosubst.syntax Undecidability.SystemF.Autosubst.unscoped Undecidability.SystemF.Util.Facts ssreflect ssrbool ssrfun