∀ {X : AlgebraicGeometry.Scheme} {M N K : X.Modules} {U : X.Opens} (φ : M ⟶ N) (ψ : N ⟶ K),
  CategoryTheory.CategoryStruct.comp (AlgebraicGeometry.Scheme.Modules.Hom.app φ U)
      (AlgebraicGeometry.Scheme.Modules.Hom.app ψ U) =
    AlgebraicGeometry.Scheme.Modules.Hom.app (CategoryTheory.CategoryStruct.comp φ ψ) U