∀ {X : Type u_1} {Y : Type u_2} {Z : Type u_3} [inst : DiffeologicalSpace X] [inst_1 : DiffeologicalSpace Y]
  [inst_2 : DiffeologicalSpace Z] {f : X → Y} {g : Y → Z},
  Diffeology.DSmooth g → Diffeology.DSmooth f → Diffeology.DSmooth fun x => g (f x)