/-- The Ping-Pong-Lemma.

Given a group action of `G` on `X` so that the generators of the free groups act in specific
ways on disjoint subsets `X i` and `Y i` we can prove that `lift f` is injective, and thus the image
of `lift f` is isomorphic to the free group.

Often the Ping-Pong-Lemma is stated with regard to group elements that generate the whole group;
we generalize to arbitrary group homomorphisms from the free group to `G` and do not require the
group to be generated by the elements.
-/
theorem _root_.FreeGroup.injective_lift_of_ping_pong : Function.Injective (FreeGroup.lift a) := by
  -- Step one: express the free group lift via the free product lift
  have : FreeGroup.lift a =
      (CoprodI.lift fun i => FreeGroup.lift fun _ => a i).comp
        (@freeGroupEquivCoprodI ι).toMonoidHom := by
    ext i
    simp
  rw [this, MonoidHom.coe_comp]
  clear this
  refine' Function.Injective.comp _ (MulEquiv.injective freeGroupEquivCoprodI)
  -- Step two: Invoke the ping-pong lemma for free products
  show Function.Injective (lift fun i : ι => FreeGroup.lift fun _ => a i)
  -- Prepare to instantiate lift_injective_of_ping_pong
  let H : ι → Type _ := fun _i => FreeGroup Unit
  let f : ∀ i, H i →* G := fun i => FreeGroup.lift fun _ => a i
  let X' : ι → Set α := fun i => X i ∪ Y i
  apply lift_injective_of_ping_pong f _ X'
  show ∀ i, (X' i).Nonempty
  · exact fun i => Set.Nonempty.inl (hXnonempty i)
  

Proof state:
case hXdisj
ι : Type u_1
M : ι → Type u_2
inst✝⁴ : (i : ι) → Monoid (M i)
N : Type u_3
inst✝³ : Monoid N
inst✝² : Nontrivial ι
G : Type u_1
inst✝¹ : Group G
a : ι → G
α : Type u_4
inst✝ : MulAction G α
X Y : ι → Set α
hXnonempty : ∀ (i : ι), Set.Nonempty (X i)
hXdisj : Pairwise fun i j => Disjoint (X i) (X j)
hYdisj : Pairwise fun i j => Disjoint (Y i) (Y j)
hXYdisj : ∀ (i j : ι), Disjoint (X i) (Y j)
hX : ∀ (i : ι), a i • (Y i)ᶜ ⊆ X i
hY : ∀ (i : ι), a⁻¹ i • (X i)ᶜ ⊆ Y i
H : ι → Type := fun _i => FreeGroup Unit
f : (i : ι) → H i →* G := fun i => FreeGroup.lift fun x => a i
X' : ι → Set α := fun i => X i ∪ Y i
⊢ Pairwise fun i j => Disjoint (X' i) (X' j)
case hpp
ι : Type u_1
M : ι → Type u_2
inst✝⁴ : (i : ι) → Monoid (M i)
N : Type u_3
inst✝³ : Monoid N
inst✝² : Nontrivial ι
G : Type u_1
inst✝¹ : Group G
a : ι → G
α : Type u_4
inst✝ : MulAction G α
X Y : ι → Set α
hXnonempty : ∀ (i : ι), Set.Nonempty (X i)
hXdisj : Pairwise fun i j => Disjoint (X i) (X j)
hYdisj : Pairwise fun i j => Disjoint (Y i) (Y j)
hXYdisj : ∀ (i j : ι), Disjoint (X i) (Y j)
hX : ∀ (i : ι), a i • (Y i)ᶜ ⊆ X i
hY : ∀ (i : ι), a⁻¹ i • (X i)ᶜ ⊆ Y i
H : ι → Type := fun _i => FreeGroup Unit
f : (i : ι) → H i →* G := fun i => FreeGroup.lift fun x => a i
X' : ι → Set α := fun i => X i ∪ Y i
⊢ Pairwise fun i j => ∀ (h : H i), h ≠ 1 → (f i) h • X' j ⊆ X' i
ι : Type u_1
M : ι → Type u_2
inst✝⁴ : (i : ι) → Monoid (M i)
N : Type u_3
inst✝³ : Monoid N
inst✝² : Nontrivial ι
G : Type u_1
inst✝¹ : Group G
a : ι → G
α : Type u_4
inst✝ : MulAction G α
X Y : ι → Set α
hXnonempty : ∀ (i : ι), Set.Nonempty (X i)
hXdisj : Pairwise fun i j => Disjoint (X i) (X j)
hYdisj : Pairwise fun i j => Disjoint (Y i) (Y j)
hXYdisj : ∀ (i j : ι), Disjoint (X i) (Y j)
hX : ∀ (i : ι), a i • (Y i)ᶜ ⊆ X i
hY : ∀ (i : ι), a⁻¹ i • (X i)ᶜ ⊆ Y i
H : ι → Type := fun _i => FreeGroup Unit
f : (i : ι) → H i →* G := fun i => FreeGroup.lift fun x => a i
X' : ι → Set α := fun i => X i ∪ Y i
⊢ 3 ≤ #ι ∨ ∃ i, 3 ≤ #(H i)

Next tactic:
show Pairwise fun i j => Disjoint (X' i) (X' j)