∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] (X : CategoryTheory.SimplicialObject C) {n : ℕ}
  {i : Fin (n + 2)},
  CategoryTheory.CategoryStruct.comp (X.δ i.castSucc) (X.δ i) = CategoryTheory.CategoryStruct.comp (X.δ i.succ) (X.δ i)