∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : CategoryTheory.HasExt C] {X : C},
  CategoryTheory.Injective X ↔ ∀ ⦃Y : C⦄, Subsingleton (CategoryTheory.Abelian.Ext Y X 1)