∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : CategoryTheory.HasExt C] (X : C) (n : ℕ) [hX : CategoryTheory.HasProjectiveDimensionLT X n] (i : ℕ),
  n ≤ i → ∀ (Y : C), Subsingleton (CategoryTheory.Abelian.Ext X Y i)