-- Lean 4 proof step from Mathlib.Analysis.CStarAlgebra.Unitization (goal)
Goal:
𝕜 : Type u_1
E : Type u_2
inst : DenselyNormedField 𝕜
inst_1 : NonUnitalNormedRing E
inst_2 : StarRing E
inst_3 : CStarRing E
inst_4 : NormedSpace 𝕜 E
inst_5 : IsScalarTower 𝕜 E E
inst_6 : SMulCommClass 𝕜 E E
inst_7 : StarRing 𝕜
inst_8 : StarModule 𝕜 E
inst_9 : CStarRing 𝕜
x : Unitization 𝕜 E
h₁ :
  ∀ (x : Unitization 𝕜 E),
    norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2 ≤
      norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x⋆).2
h₂ :
  norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) (x⋆ * x)).2 =
    HPow.hPow (β := ℕ) (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2) 2
h₃ :
  norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) (x⋆ * x)).1 =
    HPow.hPow (β := ℕ) (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1) 2
pos :
  ∀
    (h :
      norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1 ≤
        norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2),
    max (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1)
          (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2) *
        max (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1)
          (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2) ≤
      max (HPow.hPow (β := ℕ) (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1) 2)
        (HPow.hPow (β := ℕ) (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2)
          2)
⊢ max (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1)
        (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2) *
      max (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1)
        (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2) ≤
    max (HPow.hPow (β := ℕ) (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1) 2)
      (HPow.hPow (β := ℕ) (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2)
        2)
have neg : ∀
  (h :
    norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2 <
      norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1),
  max (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1)
        (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2) *
      max (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1)
        (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2) ≤
    max (HPow.hPow (β := ℕ) (norm.{u_1} (E := 𝕜) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).1) 2)
      (HPow.hPow (β := ℕ) (norm.{u_2} (E := E →L[𝕜] E) ((splitMul 𝕜 E : (a : Unitization 𝕜 E) → 𝕜 × (E →L[𝕜] E)) x).2)
        2)
Tactic: by_cases! h : ‖(Unitization.splitMul 𝕜 E x).fst‖ ≤ ‖(Unitization.splitMul 𝕜 E x).snd‖