∀ {α : Type u_1} {β : Type u_2} {γ : Type u_3} (x : Option α) (f : α → β) (g : (a : β) → Option.map f x = some a → γ),
  Std.Internal.List.Option.dmap✝ (Option.map f x) g = Std.Internal.List.Option.dmap✝¹ x fun a h => g (f a) ⋯