∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {P : Prop} [inst_1 : Decidable P] {X Y Z : C} (f : X ⟶ Y)
  (g g' : Y ⟶ Z),
  CategoryTheory.CategoryStruct.comp f (if P then g else g') =
    if P then CategoryTheory.CategoryStruct.comp f g else CategoryTheory.CategoryStruct.comp f g'