∀ {C : Type u₁} [inst : CategoryTheory.Category.{v₁, u₁} C] {D : Type u₂} [inst_1 : CategoryTheory.Category.{v₂, u₂} D]
  {F : CategoryTheory.Functor C D},
  F.leftUnitor.core =
    (CategoryTheory.Functor.id C).coreComp F ≪≫
      CategoryTheory.Functor.isoWhiskerRight (CategoryTheory.Functor.coreId C) F.core ≪≫ F.core.leftUnitor