-- Lean 4 proof step from Mathlib.Analysis.Calculus.Deriv.MeanValue (goal)
Goal:
f : ℝ → ℝ
a : ℝ
hf : Tendsto (derivWithin f (Ioi a)) (𝓝[>] a) atTop
hcont_at_a : ContinuousWithinAt f (Ici a) a
case : ∀ (hdiff : DifferentiableWithinAt ℝ f (Ioi a) a), False
case₁ : ∀ (hdiff : HasDerivWithinAt f (derivWithin f (Ioi a) a) (Ioi a) a), False
case₂ : ∀ (hdiff : Tendsto (slope f a) (𝓝[>] a) (𝓝 (derivWithin f (Ioi a) a))), False
⊢ ¬DifferentiableWithinAt ℝ f (Ioi a) a
have case₃ : ∀ (hdiff : Tendsto (slope f a) (𝓝[>] a) (𝓝 (derivWithin f (Ioi a) a)))
  (h₀ : ∀ᶠ (b : ℝ) in 𝓝[>] a, ∀ x ∈ Ioc a b, max (derivWithin f (Ioi a) a + 1) 0 < derivWithin f (Ioi a) x), False
Tactic: case pos =>
    intro hdiff
    replace hdiff := hdiff.hasDerivWithinAt
    rw [hasDerivWithinAt_iff_tendsto_slope, Set.diff_singleton_eq_self self_notMem_Ioi] at hdiff
    have h₀ : ∀ᶠ b in 𝓝[>] a,
        ∀ x ∈ Ioc a b, max (derivWithin f (Ioi a) a + 1) 0 < derivWithin f (Ioi a) x := by
      rw [(nhdsGT_basis a).eventually_iff]
      rw [(nhdsGT_basis a).tendsto_left_iff] at hf
      obtain ⟨b, hab, hb⟩ := hf (Ioi (max (derivWithin f (Ioi a) a + 1) 0)) (Ioi_mem_atTop _)
      refine ⟨b, hab, fun x hx z hz => ?_⟩
      simp only [MapsTo, mem_Ioo, mem_Ioi, and_imp] at hb
      exact hb hz.1 <| hz.2.trans_lt hx.2
    have h₁ : ∀ᶠ b in 𝓝[>] a, slope f a b < derivWithin f (Ioi a) a + 1 := by
      rw [(nhds_basis_Ioo _).tendsto_right_iff] at hdiff
      specialize hdiff ⟨derivWithin f (Ioi a) a - 1, derivWithin f (Ioi a) a + 1⟩ <| by simp
      filter_upwards [hdiff] with z hz using hz.2
    have hcontra : ∀ᶠ _ in 𝓝[>] a, False := by
      filter_upwards [h₀, h₁, eventually_mem_nhdsWithin] with b hb hslope (hab : a < b)
      have hdiff' : DifferentiableOn ℝ f (Ioc a b) := fun z hz => by
        refine DifferentiableWithinAt.mono (t := Ioi a) ?_ Ioc_subset_Ioi_self
        have : derivWithin f (Ioi a) z ≠ 0 := ne_of_gt <| by
          simp_all only [and_imp, mem_Ioc, max_lt_iff]
        exact differentiableWithinAt_of_derivWithin_ne_zero this
      have hcont_Ioc : ∀ z ∈ Ioc a b, ContinuousWithinAt f (Icc a b) z := by
        intro z hz''
        refine (hdiff'.continuousOn z hz'').mono_of_mem_nhdsWithin ?_
        have hfinal : 𝓝[Ioc a b] z = 𝓝[Icc a b] z := by
          refine nhdsWithin_eq_nhdsWithin' (s := Ioi a) (Ioi_mem_nhds hz''.1) ?_
          simp only [Ioc_inter_Ioi, le_refl, sup_of_le_left]
          ext y
          exact ⟨fun h => ⟨mem_Icc_of_Ioc h, mem_of_mem_inter_left h⟩, fun ⟨H1, H2⟩ => ⟨H2, H1.2⟩⟩
        rw [← hfinal]
        exact self_mem_nhdsWithin
      have hcont : ContinuousOn f (Icc a b) := by
        intro z hz
        by_cases hz' : z = a
        · rw [hz']
          exact hcont_at_a.mono Icc_subset_Ici_self
        · exact hcont_Ioc z ⟨lt_of_le_of_ne hz.1 (Ne.symm hz'), hz.2⟩
      obtain ⟨x, hx₁, hx₂⟩ :=
        exists_deriv_eq_slope' f hab hcont (hdiff'.mono (Ioo_subset_Ioc_self))
      specialize hb x ⟨hx₁.1, le_of_lt hx₁.2⟩
      replace hx₂ : derivWithin f (Ioi a) x = slope f a b := by
        have : Ioi a ∈ 𝓝 x := by simp [← mem_interior_iff_mem_nhds, hx₁.1]
        rwa [derivWithin_of_mem_nhds this]
      rw [hx₂, max_lt_iff] at hb
      linarith
    simp [Filter.eventually_false_iff_eq_bot, ← notMem_closure_iff_nhdsWithin_eq_bot] at hcontra