∀ {C : Type u} {X : CategoryTheory.FreeMonoidalCategory C},
  CategoryTheory.FreeMonoidalCategory.HomEquiv
    ((CategoryTheory.FreeMonoidalCategory.Hom.l_inv X).comp (CategoryTheory.FreeMonoidalCategory.Hom.l_hom X))
    (CategoryTheory.FreeMonoidalCategory.Hom.id X)