∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {X Y : C} (f : X ⟶ Y) [inst_1 : CategoryTheory.Mono f],
  CategoryTheory.CategoryStruct.comp (CategoryTheory.Subobject.underlyingIso f).hom f =
    (CategoryTheory.Subobject.mk f).arrow