∀ {R S : AddGrpCat} (f : R ⟶ S) (x : ↑((CategoryTheory.forget₂ AddGrpCat AddMonCat).obj R)),
  (CategoryTheory.ConcreteCategory.hom ((CategoryTheory.forget₂ AddGrpCat AddMonCat).map f)) x =
    (CategoryTheory.ConcreteCategory.hom f) x