∀ {X Y : AlgebraicGeometry.Scheme} (f : X ⟶ Y) (U : Y.Opens) (x : ↥X) (hx : f x ∈ U)
  (y : ↑(Y.presheaf.obj (Opposite.op U))),
  (CategoryTheory.ConcreteCategory.hom (AlgebraicGeometry.Scheme.Hom.stalkMap f x))
      ((CategoryTheory.ConcreteCategory.hom (Y.presheaf.germ U (f x) hx)) y) =
    (CategoryTheory.ConcreteCategory.hom (X.presheaf.germ ((TopologicalSpace.Opens.map f.base).obj U) x hx))
      ((CategoryTheory.ConcreteCategory.hom (AlgebraicGeometry.Scheme.Hom.app f U)) y)