∀ {X Y : AlgebraicGeometry.Scheme} (f : X ⟶ Y) (x x' : ↥X) (hxx' : x = x') {Z : CommRingCat}
  (h : X.presheaf.stalk x' ⟶ Z),
  CategoryTheory.CategoryStruct.comp (AlgebraicGeometry.Scheme.Hom.stalkMap f x)
      (CategoryTheory.CategoryStruct.comp (X.presheaf.stalkCongr ⋯).hom h) =
    CategoryTheory.CategoryStruct.comp (Y.presheaf.stalkCongr ⋯).hom
      (CategoryTheory.CategoryStruct.comp (AlgebraicGeometry.Scheme.Hom.stalkMap f x') h)