∀ {𝕜 : Type u_1} {E : Type u_2} [inst : NontriviallyNormedField 𝕜] [inst_1 : NormedAddCommGroup E]
  [inst_2 : NormedSpace 𝕜 E] {s : Set E} {x y : E},
  y ∈ tangentConeAt 𝕜 s x ↔
    ∃ c d,
      Filter.Tendsto (fun x => ‖c x‖) Filter.atTop Filter.atTop ∧
        (∀ᶠ (n : ℕ) in Filter.atTop, x + d n ∈ s) ∧ Filter.Tendsto (fun n => c n • d n) Filter.atTop (nhds y)