∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {X Y : C} {f g : X ⟶ Y}
  [inst_1 : CategoryTheory.Limits.HasEqualizer f g] {W : C} (k : W ⟶ X),
  CategoryTheory.CategoryStruct.comp k f = CategoryTheory.CategoryStruct.comp k g →
    ∃! l, CategoryTheory.CategoryStruct.comp l (CategoryTheory.Limits.equalizer.ι f g) = k