-- Lean 4 proof step from Mathlib.NumberTheory.NumberField.CanonicalEmbedding.ConvexBody (hypothesis)
Goal:
K : Type u_1
inst : Field K
inst_1 : NumberField K
I : (FractionalIdeal (R := 𝓞 K) (𝓞 K)⁰ K)ˣ
B : ℝ
h : minkowskiBound K I ≤ ((volume : Measure (mixedSpace K)) : (a : Set (mixedSpace K)) → ℝ≥0∞) (convexBodySum K B)
hB : 0 ≤ B
h1 : LT.lt (α := ℝ) 0 (↑(finrank ℚ K) : ℝ)⁻¹
h2 : 0 ≤ B / (↑(finrank ℚ K) : ℝ)
h_fund :
  IsAddFundamentalDomain.{u_1, u_1}
    ↥(toAddSubgroup (module_M := AddCommGroup.toIntModule (mixedSpace K))
        (span ℤ
          (Set.range (ι := Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K))
            (⇑(fractionalIdealLatticeBasis K I :
                  Basis (Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) ℝ
                    (mixedSpace K)) :
              (a : Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) → mixedSpace K))))
    (α := mixedSpace K)
    (fundamentalDomain (ι := Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) (K := ℝ)
      (fractionalIdealLatticeBasis K I))
    volume
this :
  Countable.{u_1 + 1}
    ↥(toAddSubgroup (module_M := AddCommGroup.toIntModule (mixedSpace K))
        (span ℤ
          (Set.range (ι := Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K))
            (⇑(fractionalIdealLatticeBasis K I :
                  Basis (Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) ℝ
                    (mixedSpace K)) :
              (a : Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) → mixedSpace K))))
x : mixedSpace K
hx :
  Membership.mem (γ := AddSubgroup (mixedSpace K))
    (toAddSubgroup (module_M := AddCommGroup.toIntModule (mixedSpace K))
      (span ℤ
        (Set.range (ι := Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K))
          (⇑(fractionalIdealLatticeBasis K I :
                Basis (Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) ℝ
                  (mixedSpace K)) :
            (a : Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) → mixedSpace K))))
    x
h_nz :
  Subtype.mk (p := fun (x : mixedSpace K) ↦
      Membership.mem (γ := AddSubgroup (mixedSpace K))
        (toAddSubgroup (module_M := AddCommGroup.toIntModule (mixedSpace K))
          (span ℤ
            (Set.range (ι := Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K))
              (⇑(fractionalIdealLatticeBasis K I :
                    Basis (Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) ℝ
                      (mixedSpace K)) :
                (a : Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) → mixedSpace K))))
        x)
      x hx ≠
    0
h_mem :
  Membership.mem (γ := Set (mixedSpace K)) (convexBodySum K B)
    (↑(Subtype.mk (p := fun (x : mixedSpace K) ↦
          Membership.mem (γ := AddSubgroup (mixedSpace K))
            (toAddSubgroup (module_M := AddCommGroup.toIntModule (mixedSpace K))
              (span ℤ
                (Set.range (ι := Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K))
                  (⇑(fractionalIdealLatticeBasis K I :
                        Basis (Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) ℝ
                          (mixedSpace K)) :
                    (a : Free.ChooseBasisIndex ℤ ↥(↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Submodule (𝓞 K) K)) →
                      mixedSpace K))))
            x)
          x hx) :
      mixedSpace K)
⊢ ∃ a ∈ (↑I : FractionalIdeal (𝓞 K)⁰ K),
    a ≠ 0 ∧ (↑|(Algebra.norm ℚ : (a : K) → ℚ) a| : ℝ) ≤ (B / (↑(finrank ℚ K) : ℝ)) ^ finrank ℚ K
have hx₁ : Membership.mem (γ := Set (mixedSpace K))
  ((⇑(mixedEmbedding K) : (a : K) → mixedSpace K) '' (↑(↑I : FractionalIdeal (𝓞 K)⁰ K) : Set K)) x
Tactic: rw [mem_toAddSubgroup, mem_span_fractionalIdealLatticeBasis] at hx
Cleared hypotheses: h_nz, h_mem