∀ {X : SSet.Truncated 1} {n : ℕ} (self : X.Path₁ n) (i : Fin n),
  X.map
      (SimplexCategory.Truncated.Hom.tr (SimplexCategory.δ 1) SSet.Truncated.Path₁._proof_1
          SSet.Truncated.Path₁._proof_5).op
      (self.arrow i) =
    self.vertex i.castSucc