/-- In a real vector space of dimension `> 1`, the complement of any countable set is path
connected. -/
theorem Set.Countable.isPathConnected_compl_of_one_lt_rank
    (h : 1 < Module.rank ℝ E) {s : Set E} (hs : s.Countable) :
    IsPathConnected sᶜ := by
  have : Nontrivial E := (rank_pos_iff_nontrivial (R := ℝ)).1 (zero_lt_one.trans h)
  -- the set `sᶜ` is dense, therefore nonempty. Pick `a ∈ sᶜ`. We have to show that any
  -- `b ∈ sᶜ` can be joined to `a`.
  obtain ⟨a, ha⟩ : sᶜ.Nonempty := (hs.dense_compl ℝ).nonempty
  refine ⟨a, ha, ?_⟩
  intro b hb
  rcases eq_or_ne a b with rfl|hab
  · exact JoinedIn.refl ha
  /- Assume `b ≠ a`. Write `a = c - x` and `b = c + x` for some nonzero `x`. Choose `y` which
  is linearly independent from `x`. Then the segments joining `a = c - x` to `c + ty` are pairwise
  disjoint for varying `t` (except for the endpoint `a`) so only countably many of them can
  intersect `s`. In the same way, there are countably many `t`s for which the segment
  from `b = c + x` to `c + ty` intersects `s`. Choosing `t` outside of these countable exceptions,
  one gets a path in the complement of `s` from `a` to `z = c + ty` and then to `b`.
  -/
  let c := (2 : ℝ)⁻¹ • (a + b)
  let x := (2 : ℝ)⁻¹ • (b - a)
  have Ia : c - x = a := by
    simp only [smul_add, smul_sub]
    abel_nf
    simp [zsmul_eq_smul_cast ℝ 2]
  have Ib : c + x = b := by
    simp only [smul_add, smul_sub]
    abel_nf
    simp [zsmul_eq_smul_cast ℝ 2]
  have x_ne_zero : x ≠ 0 := by simpa using sub_ne_zero.2 hab.symm
  obtain ⟨y, hy⟩ : ∃ y, LinearIndependent ℝ ![x, y] :=
    exists_linearIndependent_pair_of_one_lt_rank h x_ne_zero
  have A : Set.Countable {t : ℝ | ([c + x -[ℝ] c + t • y] ∩ s).Nonempty} := by
    apply countable_setOf_nonempty_of_disjoint _ (fun t ↦ inter_subset_right _ _) hs
    intro t t' htt'
    apply disjoint_iff_inter_eq_empty.2
    have N : {c + x} ∩ s = ∅ := by
      simpa only [singleton_inter_eq_empty, mem_compl_iff, Ib] using hb
    rw [inter_assoc, inter_comm s, inter_assoc, inter_self, ← inter_assoc, ← subset_empty_iff, ← N]
    apply inter_subset_inter_left
    apply Eq.subset
    apply segment_inter_eq_endpoint_of_linearIndependent_of_ne hy htt'.symm
  have B : Set.Countable {t : ℝ | ([c - x -[ℝ] c + t • y] ∩ s).Nonempty} := by
    apply countable_setOf_nonempty_of_disjoint _ (fun t ↦ inter_subset_right _ _) hs
    intro t t' htt'
    apply disjoint_iff_inter_eq_empty.2
    have N : {c - x} ∩ s = ∅ := by
      simpa only [singleton_inter_eq_empty, mem_compl_iff, Ia] using ha
    rw [inter_assoc, inter_comm s, inter_assoc, inter_self, ← inter_assoc, ← subset_empty_iff, ← N]
    apply inter_subset_inter_left
    rw [sub_eq_add_neg _ x]
    apply Eq.subset
    apply segment_inter_eq_endpoint_of_linearIndependent_of_ne _ htt'.symm
    convert hy.units_smul ![-1, 1]
    simp [← List.ofFn_inj]
  obtain ⟨t, ht⟩ : Set.Nonempty ({t : ℝ | ([c + x -[ℝ] c + t • y] ∩ s).Nonempty}
      ∪ {t : ℝ | ([c - x -[ℝ] c + t • y] ∩ s).Nonempty})ᶜ := ((A.union B).dense_compl ℝ).nonempty
  let z := c + t • y
  simp only [compl_union, mem_inter_iff, mem_compl_iff, mem_setOf_eq, not_nonempty_iff_eq_empty]
    at ht
  have JA : JoinedIn sᶜ a z := by
    apply JoinedIn.of_segment_subset
    rw [subset_compl_iff_disjoint_right, disjoint_iff_inter_eq_empty]
    convert ht.2
    exact Ia.symm
  have JB : JoinedIn sᶜ b z := by
    apply JoinedIn.of_segment_subset
    rw [subset_compl_iff_disjoint_right, disjoint_iff_inter_eq_empty]
    convert ht.1
    exact Ib.symm
  

Proof state:
case intro.inr.intro.intro
E : Type u_1
inst✝⁴ : AddCommGroup E
inst✝³ : Module ℝ E
inst✝² : TopologicalSpace E
inst✝¹ : ContinuousAdd E
inst✝ : ContinuousSMul ℝ E
h : 1 < Module.rank ℝ E
s : Set E
hs : Set.Countable s
this : Nontrivial E
a : E
ha : a ∈ sᶜ
b : E
hb : b ∈ sᶜ
hab : a ≠ b
c : E := 2⁻¹ • (a + b)
x : E := 2⁻¹ • (b - a)
Ia : c - x = a
Ib : c + x = b
x_ne_zero : x ≠ 0
y : E
hy : LinearIndependent ℝ ![x, y]
A : Set.Countable {t | Set.Nonempty ([c + x-[ℝ]c + t • y] ∩ s)}
B : Set.Countable {t | Set.Nonempty ([c - x-[ℝ]c + t • y] ∩ s)}
t : ℝ
z : E := c + t • y
ht :
  [2⁻¹ • (a + b) + 2⁻¹ • (b - a)-[ℝ]2⁻¹ • (a + b) + t • y] ∩ s = ∅ ∧
    [2⁻¹ • (a + b) - 2⁻¹ • (b - a)-[ℝ]2⁻¹ • (a + b) + t • y] ∩ s = ∅
JA : JoinedIn sᶜ a z
JB : JoinedIn sᶜ b z
⊢ JoinedIn sᶜ a b

Next tactic:
exact JA.trans JB.symm