∀ {X : SSet} {n : ℕ} {x : X.obj (Opposite.op { len := 0 })} {f g : X.PtSimplex n x} {i : Fin (n + 1)}
  (self : f.RelStruct g i) {Z : SSet} (h : X ⟶ Z),
  CategoryTheory.CategoryStruct.comp (SSet.stdSimplex.δ i.succ) (CategoryTheory.CategoryStruct.comp self.map h) =
    CategoryTheory.CategoryStruct.comp g.map h