theorem Zlattice.rank : finrank ℤ L = finrank K E := by
  classical
  have : Module.Finite ℤ L := module_finite K hs
  have : Module.Free ℤ L := module_free K hs
  have : Module ℚ E := Module.compHom E (algebraMap ℚ K)
  let b₀ := Module.Free.chooseBasis ℤ L
  -- Let `b` be a `ℤ`-basis of `L` formed of vectors of `E`
  let b := Subtype.val ∘ b₀
  have : LinearIndependent ℤ b :=
    LinearIndependent.map' b₀.linearIndependent (L.toIntSubmodule.subtype) (ker_subtype _)
  -- We prove some assertions that will be useful later on
  have h_spanL : span ℤ (Set.range b) = AddSubgroup.toIntSubmodule L := by
    convert congrArg (map (Submodule.subtype (AddSubgroup.toIntSubmodule L))) b₀.span_eq
    · rw [map_span, Set.range_comp]
      rfl
    · exact (map_subtype_top _).symm
  have h_spanE : span K (Set.range b) = ⊤ := by rwa [← span_span_of_tower (R := ℤ), h_spanL]
  have h_card : Fintype.card (Module.Free.ChooseBasisIndex ℤ L) =
      (Set.range b).toFinset.card := by
    rw [Set.toFinset_range, Finset.univ.card_image_of_injective]
    rfl
    exact Subtype.coe_injective.comp (Basis.injective _)
  rw [finrank_eq_card_chooseBasisIndex]
    -- We prove that `finrank ℤ L ≤ finrank K E` and `finrank K E ≤ finrank ℤ L`
  refine le_antisymm ?_ ?_
  · -- To prove that `finrank ℤ L ≤ finrank K E`, we proceed by contradiction and prove that, in
    -- this case, there is a ℤ-relation between the vectors of `b`
    obtain ⟨t, ⟨ht_inc, ⟨ht_span, ht_lin⟩⟩⟩ := exists_linearIndependent K (Set.range b)
    -- `e` is a `K`-basis of `E` formed of vectors of `b`
    let e : Basis t K E := Basis.mk ht_lin (by simp [ht_span, h_spanE])
    have : Fintype t := Set.Finite.fintype ((Set.range b).toFinite.subset ht_inc)
    have h : LinearIndependent ℤ (fun x : (Set.range b) => (x : E)) := by
      rwa [linearIndependent_subtype_range (Subtype.coe_injective.comp b₀.injective)]
    contrapose! h
    -- Since `finrank ℤ L > finrank K E`, there exists a vector `v ∈ b` with `v ∉ e`
    obtain ⟨v, hv⟩ : (Set.range b \ Set.range e).Nonempty := by
      rw [Basis.coe_mk, Subtype.range_coe_subtype, Set.setOf_mem_eq, ← Set.toFinset_nonempty]
      contrapose h
      rw [Finset.not_nonempty_iff_eq_empty, Set.toFinset_diff,
        Finset.sdiff_eq_empty_iff_subset] at h
      replace h := Finset.card_le_of_subset h
      rwa [not_lt, h_card, ← topEquiv.finrank_eq, ← h_spanE, ← ht_span,
        finrank_span_set_eq_card _ ht_lin]
    -- Assume that `e ∪ {v}` is not `ℤ`-linear independent then we get the contradiction
    suffices ¬ LinearIndependent ℤ (fun x : ↥(insert v (Set.range e)) => (x : E)) by
      contrapose! this
      refine LinearIndependent.mono ?_ this
      

Proof state:
K : Type u_1
inst✝⁷ : NormedLinearOrderedField K
inst✝⁶ : HasSolidNorm K
inst✝⁵ : FloorRing K
E : Type u_2
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace K E
inst✝² : FiniteDimensional K E
inst✝¹ : ProperSpace E
L : AddSubgroup E
inst✝ : DiscreteTopology ↥L
hs : span K ↑L = ⊤
this✝⁴ : Module.Finite ℤ ↥L
this✝³ : Module.Free ℤ ↥L
this✝² : Module ℚ E
b₀ : Basis (Module.Free.ChooseBasisIndex ℤ ↥L) ℤ ↥L := Module.Free.chooseBasis ℤ ↥L
b : Module.Free.ChooseBasisIndex ℤ ↥L → E := Subtype.val ∘ ⇑b₀
this✝¹ : LinearIndependent ℤ b
h_spanL : span ℤ (Set.range b) = AddSubgroup.toIntSubmodule L
h_spanE : span K (Set.range b) = ⊤
h_card : Fintype.card (Module.Free.ChooseBasisIndex ℤ ↥L) = Finset.card (Set.toFinset (Set.range b))
t : Set E
ht_inc : t ⊆ Set.range b
ht_span : span K t = span K (Set.range b)
ht_lin : LinearIndependent (ι := { x // x ∈ t }) K Subtype.val
e : Basis (↑t) K E := Basis.mk ht_lin (_ : ⊤ ≤ span K (Set.range Subtype.val))
this✝ : Fintype ↑t
h : finrank K E < Fintype.card (Module.Free.ChooseBasisIndex ℤ ↥L)
v : E
hv : v ∈ Set.range b \ Set.range ⇑e
this : LinearIndependent ℤ fun (x : ↑(Set.range b)) => ↑x
⊢ insert v (Set.range ⇑e) ⊆ Set.range b

Next tactic:
exact Set.insert_subset (Set.mem_of_mem_diff hv) (by simp [ht_inc])