∀ {T : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} T] {X : T} {f g : CategoryTheory.Over X} (p : f ⟶ g)
  (X_1 : CategoryTheory.Over f),
  (CategoryTheory.Over.iteratedSliceForwardNaturalityIso p).hom.app X_1 =
    CategoryTheory.CategoryStruct.id ((CategoryTheory.Over.map p.left).obj (CategoryTheory.Over.mk X_1.hom.left))