{T : Type u₁} →
  [inst : CategoryTheory.Category.{v₁, u₁} T] →
    {D : Type u₂} →
      [inst_1 : CategoryTheory.Category.{v₂, u₂} D] →
        (X : T) → (F : CategoryTheory.Functor T D) → F.FullyFaithful → (CategoryTheory.Under.post F).FullyFaithful