∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [inst_1 : CategoryTheory.Category.{v₂, u₂} D]
  {E : Type u₃} [inst_2 : CategoryTheory.Category.{v₃, u₃} E] (L : CategoryTheory.Functor C D)
  (R : CategoryTheory.Functor E D) {X Y : CategoryTheory.Comma L R} (f : X ⟶ Y),
  ((CategoryTheory.CostructuredArrow.commaToGrothendieckPrecompFunctor L R).map f).fiber =
    CategoryTheory.CostructuredArrow.homMk f.left ⋯