∀ {R S : CommRingCat} [inst : Algebra ↑R ↑S] {A B : CategoryTheory.Under R} (f g : A ⟶ B),
  (CommRingCat.Under.tensorProdEqualizer f g).ι =
    (R.tensorProd S).map (AlgHom.equalizer (CommRingCat.toAlgHom f) (CommRingCat.toAlgHom g)).val.toUnder