∀ {C : Type u} [inst : CategoryTheory.Category.{v, u} C] {F : CategoryTheory.Functor C CategoryTheory.Cat}
  {X Y : CategoryTheory.Grothendieck F} (f g : X.Hom Y) (w_base : f.base = g.base),
  CategoryTheory.CategoryStruct.comp (CategoryTheory.eqToHom ⋯) f.fiber = g.fiber → f = g