∀ {α : Type u_1} {β : Type u_2} [inst : TopologicalSpace α] {a : α} {l : Filter β} {s : Set α} {f : β → α},
  Filter.Tendsto f l (nhdsWithin a s) = (Filter.Tendsto f l (nhds a) ∧ ∀ᶠ (n : β) in l, f n ∈ s)