-- Lean 4 proof step from Mathlib.Topology.TietzeExtension (goal)
Goal:
X : Type u_1
Y : Type u_2
inst : TopologicalSpace X
inst_1 : TopologicalSpace Y
inst_2 : NormalSpace Y
inst_3 : Nonempty X
f : X →ᵇ ℝ
e : X → Y
he : IsClosedEmbedding e
inhabited_h : Inhabited X
a : ℝ
ha : IsGLB (range (ι := X) (⇑f : (a : X) → ℝ)) a
b : ℝ
hb : IsLUB (range (ι := X) (⇑f : (a : X) → ℝ)) b
hmem : ∀ (x : X), Membership.mem (α := ℝ) (Icc a b) ((f : (a : X) → ℝ) x : ℝ)
hle : a ≤ b
hlt : a < b
c : ℝ := (a + b) / 2
hac : a < c
hcb : c < b
hsub : c - a = b - c
⊢ ∃ (g : Y →ᵇ ℝ),
    (∀ (y : Y),
        ∃ (x₁ : X) (x₂ : X),
          Membership.mem (α := ℝ) (γ := Set ℝ) (Icc ((f : (a : X) → ℝ) x₁) ((f : (a : X) → ℝ) x₂) : Set ℝ)
            ((g : (a : Y) → ℝ) y : ℝ)) ∧
      Eq (α := X → ℝ) ((⇑g : (a : Y) → ℝ) ∘ e) (⇑f : (a : X) → ℝ)
have inr : ∀ (g : Y →ᵇ ℝ)
  (hg_mem :
    ∀ (y : Y),
      ∃ (x : X), Membership.mem (α := ℝ) (γ := Set ℝ) (Icc ((f : (a : X) → ℝ) x) b : Set ℝ) ((g : (a : Y) → ℝ) y : ℝ))
  (hgf : Eq (α := X → ℝ) ((⇑g : (a : Y) → ℝ) ∘ e) (⇑f : (a : X) → ℝ)),
  ∃ (g : Y →ᵇ ℝ),
    (∀ (y : Y),
        ∃ (x₁ : X) (x₂ : X),
          Membership.mem (α := ℝ) (γ := Set ℝ) (Icc ((f : (a : X) → ℝ) x₁) ((f : (a : X) → ℝ) x₂) : Set ℝ)
            ((g : (a : Y) → ℝ) y : ℝ)) ∧
      Eq (α := X → ℝ) ((⇑g : (a : Y) → ℝ) ∘ e) (⇑f : (a : X) → ℝ)
Tactic: obtain ⟨g, hg_mem, hgf⟩ : ∃ g : Y →ᵇ ℝ, (∀ y, ∃ x, g y ∈ Icc (f x) b) ∧ g ∘ e = f := by
    rcases exists_extension_forall_mem_Icc_of_isClosedEmbedding f hmem hle he with ⟨g, hg_mem, hgf⟩
    -- If `a ∈ range f`, then we are done.
    rcases em (∃ x, f x = a) with (⟨x, rfl⟩ | ha')
    · exact ⟨g, fun y => ⟨x, hg_mem _⟩, hgf⟩
    /- Otherwise, `g ⁻¹' {a}` is disjoint with `range e ∪ g ⁻¹' (Ici c)`, hence there exists a
        function `dg : Y → ℝ` such that `dg ∘ e = 0`, `dg y = 0` whenever `c ≤ g y`, `dg y = c - a`
        whenever `g y = a`, and `0 ≤ dg y ≤ c - a` for all `y`. -/
    have hd : Disjoint (range e ∪ g ⁻¹' Ici c) (g ⁻¹' {a}) := by
      refine disjoint_union_left.2 ⟨?_, Disjoint.preimage _ ?_⟩
      · rw [Set.disjoint_left]
        rintro _ ⟨x, rfl⟩ (rfl : g (e x) = a)
        exact ha' ⟨x, (congr_fun hgf x).symm⟩
      · exact Set.disjoint_singleton_right.2 hac.not_ge
    rcases exists_bounded_mem_Icc_of_closed_of_le
        (he.isClosed_range.union <| isClosed_Ici.preimage g.continuous)
        (isClosed_singleton.preimage g.continuous) hd (sub_nonneg.2 hac.le) with
      ⟨dg, dg0, dga, dgmem⟩
    replace hgf : ∀ x, (g + dg) (e x) = f x := by
      intro x
      simp [dg0 (Or.inl <| mem_range_self _), ← hgf]
    refine ⟨g + dg, fun y => ?_, funext hgf⟩
    have hay : a < (g + dg) y := by
      rcases (hg_mem y).1.eq_or_lt with (rfl | hlt)
      · refine (lt_add_iff_pos_right _).2 ?_
        calc
          0 < c - g y := sub_pos.2 hac
          _ = dg y := (dga rfl).symm
      · exact hlt.trans_le (le_add_of_nonneg_right (dgmem y).1)
    rcases ha.exists_between hay with ⟨_, ⟨x, rfl⟩, _, hxy⟩
    refine ⟨x, hxy.le, ?_⟩
    rcases le_total c (g y) with hc | hc
    · simp [dg0 (Or.inr hc), (hg_mem y).2]
    · calc
        g y + dg y ≤ c + (c - a) := add_le_add hc (dgmem _).2
        _ = b := by rw [hsub, add_sub_cancel]