∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] [inst_1 : CategoryTheory.Abelian C]
  [inst_2 : CategoryTheory.HasExt C] {X Y : C} (R : CategoryTheory.InjectiveResolution Y) {n : ℕ}
  (x y : CochainComplex.HomComplex.CohomologyClass ((CochainComplex.singleFunctor C 0).obj X) R.cochainComplex ↑n),
  R.extEquivCohomologyClass.symm (x - y) = R.extEquivCohomologyClass.symm x - R.extEquivCohomologyClass.symm y