-- Mathlib/Order/JordanHolder.lean
theorem snoc_snoc_swap {s : CompositionSeries X} {x₁ x₂ y₁ y₂ : X} {hsat₁ : IsMaximal s.last x₁}
    {hsat₂ : IsMaximal s.last x₂} {hsaty₁ : IsMaximal (snoc s x₁ hsat₁).last y₁}
    {hsaty₂ : IsMaximal (snoc s x₂ hsat₂).last y₂} (hr₁ : Iso (s.last, x₁) (x₂, y₂))
    (hr₂ : Iso (x₁, y₁) (s.last, x₂)) :
    Equivalent (snoc (snoc s x₁ hsat₁) y₁ hsaty₁) (snoc (snoc s x₂ hsat₂) y₂ hsaty₂) :=
  let e : Fin (s.length + 1 + 1) ≃ Fin (s.length + 1 + 1) :=
    Equiv.swap (Fin.last _) (Fin.castSucc (Fin.last _))
  have h1 : ∀ {i : Fin s.length},
      (Fin.castSucc (Fin.castSucc i)) ≠ (Fin.castSucc (Fin.last _)) := fun {_} =>
    ne_of_lt (by simp [Fin.castSucc_lt_last])
  have h2 : ∀ {i : Fin s.length},
      (Fin.castSucc (Fin.castSucc i)) ≠ Fin.last _ := fun {_} =>
    ne_of_lt (by simp [Fin.castSucc_lt_last])
  ⟨e, by
    intro i
    dsimp only [e]
    refine Fin.lastCases ?_ (fun i => ?_) i
    · erw [Equiv.swap_apply_left, snoc_castSucc,
      show (snoc s x₁ hsat₁).toFun (Fin.last _) = x₁ from last_snoc _ _ _, Fin.succ_last,
      show ((s.snoc x₁ hsat₁).snoc y₁ hsaty₁).toFun (Fin.last _) = y₁ from last_snoc _ _ _,
      snoc_castSucc, snoc_castSucc, Fin.succ_castSucc, snoc_castSucc, Fin.succ_last,
      show (s.snoc _ hsat₂).toFun (Fin.last _) = x₂ from last_snoc _ _ _]
      exact hr₂
    · refine Fin.lastCases ?_ (fun i => ?_) i
      · erw [Equiv.swap_apply_right, snoc_castSucc, snoc_castSucc, snoc_castSucc,
          Fin.succ_castSucc, snoc_castSucc, Fin.succ_last, last_snoc', last_snoc', last_snoc']
        exact hr₁
      · erw [Equiv.swap_apply_of_ne_of_ne h2 h1, snoc_castSucc, snoc_castSucc,
          snoc_castSucc, snoc_castSucc, Fin.succ_castSucc, snoc_castSucc,
          Fin.succ_castSucc, snoc_castSucc, snoc_castSucc, snoc_castSucc]
        exact (s.step i).iso_refl⟩

Proof state:
X : Type u
inst✝¹ : Lattice X
inst✝ : JordanHolderLattice X
s : CompositionSeries X
x₁ x₂ y₁ y₂ : X
hsat₁ : IsMaximal (last s) x₁
hsat₂ : IsMaximal (last s) x₂
hsaty₁ : IsMaximal (snoc s x₁ hsat₁).last y₁
hsaty₂ : IsMaximal (snoc s x₂ hsat₂).last y₂
hr₁ : Iso (last s, x₁) (x₂, y₂)
hr₂ : Iso (x₁, y₁) (last s, x₂)
e : Fin (s.length + 1 + 1) ≃ Fin (s.length + 1 + 1) := Equiv.swap (Fin.last (s.length + 1)) (Fin.last s.length).castSucc
h1 : ∀ {i : Fin s.length}, i.castSucc.castSucc ≠ (Fin.last s.length).castSucc
h2 : ∀ {i : Fin s.length}, i.castSucc.castSucc ≠ Fin.last (s.length + 1)
i : Fin ((snoc s x₁ hsat₁).snoc y₁ hsaty₁).length
⊢ Iso (((snoc s x₁ hsat₁).snoc y₁ hsaty₁).toFun i.castSucc, ((snoc s x₁ hsat₁).snoc y₁ hsaty₁).toFun i.succ)
    (((snoc s x₂ hsat₂).snoc y₂ hsaty₂).toFun
        ((Equiv.swap (Fin.last (s.length + 1)) (Fin.last s.length).castSucc) i).castSucc,
      ((snoc s x₂ hsat₂).snoc y₂ hsaty₂).toFun
        ((Equiv.swap (Fin.last (s.length + 1)) (Fin.last s.length).castSucc) i).succ)

Tactic:
refine Fin.lastCases ?_ (fun i => ?_) i

Result:
case refine_1
X : Type u
inst✝¹ : Lattice X
inst✝ : JordanHolderLattice X
s : CompositionSeries X
x₁ x₂ y₁ y₂ : X
hsat₁ : IsMaximal (last s) x₁
hsat₂ : IsMaximal (last s) x₂
hsaty₁ : IsMaximal (snoc s x₁ hsat₁).last y₁
hsaty₂ : IsMaximal (snoc s x₂ hsat₂).last y₂
hr₁ : Iso (last s, x₁) (x₂, y₂)
hr₂ : Iso (x₁, y₁) (last s, x₂)
e : Fin (s.length + 1 + 1) ≃ Fin (s.length + 1 + 1) := Equiv.swap (Fin.last (s.length + 1)) (Fin.last s.length).castSucc
h1 : ∀ {i : Fin s.length}, i.castSucc.castSucc ≠ (Fin.last s.length).castSucc
h2 : ∀ {i : Fin s.length}, i.castSucc.castSucc ≠ Fin.last (s.length + 1)
i : Fin ((snoc s x₁ hsat₁).snoc y₁ hsaty₁).length
⊢ Iso
    (((snoc s x₁ hsat₁).snoc y₁ hsaty₁).toFun
        (Fin.last ((snoc s x₁ hsat₁).length + (RelSeries.singleton IsMaximal y₁).length)).castSucc,
      ((snoc s x₁ hsat₁).snoc y₁ hsaty₁).toFun
        (Fin.last ((snoc s x₁ hsat₁).length + (RelSeries.singleton IsMaximal y₁).length)).succ)
    (((snoc s x₂ hsat₂).snoc y₂ hsaty₂).toFun
        ((Equiv.swap (Fin.last (s.length + 1)) (Fin.last s.length).castSucc)
            (Fin.last ((snoc s x₁ hsat₁).length + (RelSeries.singleton IsMaximal y₁).length))).castSucc,
      ((snoc s x₂ hsat₂).snoc y₂ hsaty₂).toFun
        ((Equiv.swap (Fin.last (s.length + 1)) (Fin.last s.length).castSucc)
            (Fin.last ((snoc s x₁ hsat₁).length + (RelSeries.singleton IsMaximal y₁).length))).succ)

case refine_2
X : Type u
inst✝¹ : Lattice X
inst✝ : JordanHolderLattice X
s : CompositionSeries X
x₁ x₂ y₁ y₂ : X
hsat₁ : IsMaximal (last s) x₁
hsat₂ : IsMaximal (last s) x₂
hsaty₁ : IsMaximal (snoc s x₁ hsat₁).last y₁
hsaty₂ : IsMaximal (snoc s x₂ hsat₂).last y₂
hr₁ : Iso (last s, x₁) (x₂, y₂)
hr₂ : Iso (x₁, y₁) (last s, x₂)
e : Fin (s.length + 1 + 1) ≃ Fin (s.length + 1 + 1) := Equiv.swap (Fin.last (s.length + 1)) (Fin.last s.length).castSucc
h1 : ∀ {i : Fin s.length}, i.castSucc.castSucc ≠ (Fin.last s.length).castSucc
h2 : ∀ {i : Fin s.length}, i.castSucc.castSucc ≠ Fin.last (s.length + 1)
i✝ : Fin ((snoc s x₁ hsat₁).snoc y₁ hsaty₁).length
i : Fin ((snoc s x₁ hsat₁).length + (RelSeries.singleton IsMaximal y₁).length)
⊢ Iso
    (((snoc s x₁ hsat₁).snoc y₁ hsaty₁).toFun i.castSucc.castSucc,
      ((snoc s x₁ hsat₁).snoc y₁ hsaty₁).toFun i.castSucc.succ)
    (((snoc s x₂ hsat₂).snoc y₂ hsaty₂).toFun
        ((Equiv.swap (Fin.last (s.length + 1)) (Fin.last s.length).castSucc) i.castSucc).castSucc,
      ((snoc s x₂ hsat₂).snoc y₂ hsaty₂).toFun
        ((Equiv.swap (Fin.last (s.length + 1)) (Fin.last s.length).castSucc) i.castSucc).succ)