Binding lemmas about D<:. To reduce compile times, unary_lr should not depend on this file. This file should load as little Iris code as possible, to reduce compile times.
From D.DSub Require Import syn.

Implicit Types
         (L T U : ty) (v : vl) (e : tm)
         (Γ : ctx) (ρ : vls).

Lemma tskip_subst i e s : (iterate tskip i e).|[s] = iterate tskip i e.|[s].
Proof. elim: i ⇒ [|i IHi]; by rewrite ?iterate_0 ?iterate_S //= IHi. Qed.