∀ {α : Type u_1} {β : Type u_2} {γ : Type u_3} [inst : TopologicalSpace α] [inst_1 : TopologicalSpace β]
  [inst_2 : TopologicalSpace γ] [DiscreteTopology α] {f : α × β → γ} {s : Set (α × β)} {x : α × β},
  ContinuousWithinAt f s x = ContinuousWithinAt (fun x_1 => f (x.1, x_1)) {b | (x.1, b) ∈ s} x.2