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