∀ {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} (hS : S.ShortExact) {n₀ : ℕ}
  (x₃ : CategoryTheory.Abelian.Ext X S.X₃ n₀) {n₁ : ℕ} (hn₁ : n₀ + 1 = n₁),
  x₃.comp hS.extClass hn₁ = 0 → ∃ x₂, x₂.comp (CategoryTheory.Abelian.Ext.mk₀ S.g) ⋯ = x₃