∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : CategoryTheory.HasExt C] (P Y : C) [CategoryTheory.Projective P] (n : ℕ),
  Subsingleton (CategoryTheory.Abelian.Ext P Y (n + 1))