∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] (X : CategoryTheory.SimplicialObject C) (n : ℕ),
  X.IsCoskeletal n ↔
    CategoryTheory.Functor.IsRightKanExtension X
      (CategoryTheory.CategoryStruct.id ((SimplexCategory.Truncated.inclusion n).op.comp X))