∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : CategoryTheory.HasExt C] (X : C) {S : CategoryTheory.ShortComplex C},
  S.ShortExact →
    ∀ {n : ℕ} (x₂ : CategoryTheory.Abelian.Ext X S.X₂ n),
      x₂.comp (CategoryTheory.Abelian.Ext.mk₀ S.g) ⋯ = 0 → ∃ x₁, x₁.comp (CategoryTheory.Abelian.Ext.mk₀ S.f) ⋯ = x₂