∀ {X Y : AlgebraicGeometry.Scheme} (f : X ⟶ Y) (U : Y.Opens),
  AlgebraicGeometry.Scheme.Hom.appTop (f ∣_ U) =
    CategoryTheory.CategoryStruct.comp
      (AlgebraicGeometry.Scheme.Hom.app f ((AlgebraicGeometry.Scheme.Hom.opensFunctor U.ι).obj ⊤))
      (X.presheaf.map (CategoryTheory.eqToHom ⋯).op)