-- Lean 4 proof step from Mathlib.Analysis.InnerProductSpace.OfNorm (hypothesis)
Goal:
𝕜 : Type u_1
inst : RCLike 𝕜
E : Type u_2
inst_1 : NormedAddCommGroup E
inst_2 : NormedSpace 𝕜 E
inst_3 : InnerProductSpaceable E
x y z : E
h1 :
  ‖x + y + z + (x - z)‖ * ‖x + y + z + (x - z)‖ + ‖x + y + z - (x - z)‖ * ‖x + y + z - (x - z)‖ =
    2 * (‖x + y + z‖ * ‖x + y + z‖ + ‖x - z‖ * ‖x - z‖)
h2 :
  ‖x + y - z + (x + z)‖ * ‖x + y - z + (x + z)‖ + ‖x + y - z - (x + z)‖ * ‖x + y - z - (x + z)‖ =
    2 * (‖x + y - z‖ * ‖x + y - z‖ + ‖x + z‖ * ‖x + z‖)
h3 : ‖y + z + z‖ * ‖y + z + z‖ + ‖y + z - z‖ * ‖y + z - z‖ = 2 * (‖y + z‖ * ‖y + z‖ + ‖z‖ * ‖z‖)
h4 : ‖y - z + z‖ * ‖y - z + z‖ + ‖y - z - z‖ * ‖y - z - z‖ = 2 * (‖y - z‖ * ‖y - z‖ + ‖z‖ * ‖z‖)
h5 :
  ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) + z + (HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x - z)‖ *
        ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) + z + (HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x - z)‖ +
      ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) + z - (HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x - z)‖ *
        ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) + z - (HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x - z)‖ =
    2 *
      (‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) + z‖ * ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) + z‖ +
        ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x - z‖ * ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x - z‖)
⊢ 4⁻¹ *
      (HMul.hMul (α := 𝕜) (β := 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖x + y + z‖ : 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖x + y + z‖ : 𝕜) -
            HMul.hMul (α := 𝕜) (β := 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖x + y - z‖ : 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖x + y - z‖ : 𝕜) +
          HMul.hMul (β := 𝕜)
            (HMul.hMul (α := 𝕜) (β := 𝕜) (I : 𝕜)
              ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) + z‖ : 𝕜))
            ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) + z‖ : 𝕜) -
        HMul.hMul (β := 𝕜)
          (HMul.hMul (α := 𝕜) (β := 𝕜) (I : 𝕜)
            ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) - z‖ : 𝕜))
          ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) - z‖ : 𝕜)) =
    4⁻¹ *
        (HMul.hMul (α := 𝕜) (β := 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖x + z‖ : 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖x + z‖ : 𝕜) -
              HMul.hMul (α := 𝕜) (β := 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖x - z‖ : 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖x - z‖ : 𝕜) +
            HMul.hMul (β := 𝕜)
              (HMul.hMul (α := 𝕜) (β := 𝕜) (I : 𝕜)
                ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x + z‖ : 𝕜))
              ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x + z‖ : 𝕜) -
          HMul.hMul (β := 𝕜)
            (HMul.hMul (α := 𝕜) (β := 𝕜) (I : 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x - z‖ : 𝕜))
            ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x - z‖ : 𝕜)) +
      4⁻¹ *
        (HMul.hMul (α := 𝕜) (β := 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖y + z‖ : 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖y + z‖ : 𝕜) -
              HMul.hMul (α := 𝕜) (β := 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖y - z‖ : 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖y - z‖ : 𝕜) +
            HMul.hMul (β := 𝕜)
              (HMul.hMul (α := 𝕜) (β := 𝕜) (I : 𝕜)
                ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I y + z‖ : 𝕜))
              ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I y + z‖ : 𝕜) -
          HMul.hMul (β := 𝕜)
            (HMul.hMul (α := 𝕜) (β := 𝕜) (I : 𝕜) ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I y - z‖ : 𝕜))
            ((𝓚 : (a : ℝ) → 𝕜) ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I y - z‖ : 𝕜))
have h6 : ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) - z + (HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x + z)‖ *
      ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) - z + (HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x + z)‖ +
    ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) - z - (HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x + z)‖ *
      ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) - z - (HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x + z)‖ =
  2 *
    (‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) - z‖ * ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I (x + y) - z‖ +
      ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x + z‖ * ‖HSMul.hSMul.{u_1, u_2, u_2} (α := 𝕜) I x + z‖)
Tactic: have h6 := parallelogram_identity ((I : 𝕜) • (x + y) - z) ((I : 𝕜) • x + z)