∀ {X Y Z : GrpCat} (f : X ⟶ Y) (g : Y ⟶ Z),
  MonCat.ofHom (GrpCat.Hom.hom (CategoryTheory.CategoryStruct.comp f g)) =
    CategoryTheory.CategoryStruct.comp (MonCat.ofHom (GrpCat.Hom.hom f)) (MonCat.ofHom (GrpCat.Hom.hom g))