-- Mathlib/Analysis/Analytic/Composition.lean
theorem comp_assoc (r : FormalMultilinearSeries 𝕜 G H) (q : FormalMultilinearSeries 𝕜 F G)
    (p : FormalMultilinearSeries 𝕜 E F) : (r.comp q).comp p = r.comp (q.comp p)

Proof state:
case h.H.mk
𝕜 : Type u_1
E : Type u_2
F : Type u_3
G : Type u_4
H : Type u_5
inst✝⁸ : NontriviallyNormedField 𝕜
inst✝⁷ : NormedAddCommGroup E
inst✝⁶ : NormedSpace 𝕜 E
inst✝⁵ : NormedAddCommGroup F
inst✝⁴ : NormedSpace 𝕜 F
inst✝³ : NormedAddCommGroup G
inst✝² : NormedSpace 𝕜 G
inst✝¹ : NormedAddCommGroup H
inst✝ : NormedSpace 𝕜 H
r : FormalMultilinearSeries 𝕜 G H
q : FormalMultilinearSeries 𝕜 F G
p : FormalMultilinearSeries 𝕜 E F
n : ℕ
v : Fin n → E
f : (a : Composition n) × Composition a.length → H :=
  fun c => (r c.snd.length) (q.applyComposition c.snd (p.applyComposition c.fst v))
g : (c : Composition n) × ((i : Fin c.length) → Composition (c.blocksFun i)) → H :=
  fun c => (r c.fst.length) fun i => (q (c.snd i).length) (p.applyComposition (c.snd i) (v ∘ ⇑(c.fst.embedding i)))
a : Composition n
b : Composition a.length
a✝ : ⟨a, b⟩ ∈ Finset.univ
⊢ ∀ (i : ℕ) (him : i < b.length) (hin : i < (a.gather b).length),
    q.applyComposition ⟨a, b⟩.snd (p.applyComposition ⟨a, b⟩.fst v) ⟨i, him⟩ =
      (q (⟨a.gather b, a.sigmaCompositionAux b⟩.snd ⟨i, hin⟩).length)
        (p.applyComposition (⟨a.gather b, a.sigmaCompositionAux b⟩.snd ⟨i, hin⟩)
          (v ∘ ⇑(⟨a.gather b, a.sigmaCompositionAux b⟩.fst.embedding ⟨i, hin⟩)))

Tactic:
intro i hi1 hi2

Result:
case h.H.mk
𝕜 : Type u_1
E : Type u_2
F : Type u_3
G : Type u_4
H : Type u_5
inst✝⁸ : NontriviallyNormedField 𝕜
inst✝⁷ : NormedAddCommGroup E
inst✝⁶ : NormedSpace 𝕜 E
inst✝⁵ : NormedAddCommGroup F
inst✝⁴ : NormedSpace 𝕜 F
inst✝³ : NormedAddCommGroup G
inst✝² : NormedSpace 𝕜 G
inst✝¹ : NormedAddCommGroup H
inst✝ : NormedSpace 𝕜 H
r : FormalMultilinearSeries 𝕜 G H
q : FormalMultilinearSeries 𝕜 F G
p : FormalMultilinearSeries 𝕜 E F
n : ℕ
v : Fin n → E
f : (a : Composition n) × Composition a.length → H :=
  fun c => (r c.snd.length) (q.applyComposition c.snd (p.applyComposition c.fst v))
g : (c : Composition n) × ((i : Fin c.length) → Composition (c.blocksFun i)) → H :=
  fun c => (r c.fst.length) fun i => (q (c.snd i).length) (p.applyComposition (c.snd i) (v ∘ ⇑(c.fst.embedding i)))
a : Composition n
b : Composition a.length
a✝ : ⟨a, b⟩ ∈ Finset.univ
i : ℕ
hi1 : i < b.length
hi2 : i < (a.gather b).length
⊢ q.applyComposition ⟨a, b⟩.snd (p.applyComposition ⟨a, b⟩.fst v) ⟨i, hi1⟩ =
    (q (⟨a.gather b, a.sigmaCompositionAux b⟩.snd ⟨i, hi2⟩).length)
      (p.applyComposition (⟨a.gather b, a.sigmaCompositionAux b⟩.snd ⟨i, hi2⟩)
        (v ∘ ⇑(⟨a.gather b, a.sigmaCompositionAux b⟩.fst.embedding ⟨i, hi2⟩)))

Proof state:
case h.H.mk
𝕜 : Type u_1
E : Type u_2
F : Type u_3
G : Type u_4
H : Type u_5
inst✝⁸ : NontriviallyNormedField 𝕜
inst✝⁷ : NormedAddCommGroup E
inst✝⁶ : NormedSpace 𝕜 E
inst✝⁵ : NormedAddCommGroup F
inst✝⁴ : NormedSpace 𝕜 F
inst✝³ : NormedAddCommGroup G
inst✝² : NormedSpace 𝕜 G
inst✝¹ : NormedAddCommGroup H
inst✝ : NormedSpace 𝕜 H
r : FormalMultilinearSeries 𝕜 G H
q : FormalMultilinearSeries 𝕜 F G
p : FormalMultilinearSeries 𝕜 E F
n : ℕ
v : Fin n → E
f : (a : Composition n) × Composition a.length → H :=
  fun c => (r c.snd.length) (q.applyComposition c.snd (p.applyComposition c.fst v))
g : (c : Composition n) × ((i : Fin c.length) → Composition (c.blocksFun i)) → H :=
  fun c => (r c.fst.length) fun i => (q (c.snd i).length) (p.applyComposition (c.snd i) (v ∘ ⇑(c.fst.embedding i)))
a : Composition n
b : Composition a.length
a✝ : ⟨a, b⟩ ∈ Finset.univ
i : ℕ
hi1 : i < b.length
hi2 : i < (a.gather b).length
j : ℕ
hj1 : j < b.blocksFun ⟨i, hi1⟩
hj2 : j < (a.sigmaCompositionAux b ⟨↑⟨i, hi1⟩, ⋯⟩).length
k : ℕ
hk1 : k < a.blocksFun ((b.embedding ⟨i, hi1⟩) ⟨j, hj1⟩)
hk2 : k < (a.sigmaCompositionAux b ⟨↑⟨i, hi1⟩, ⋯⟩).blocksFun ⟨↑⟨j, hj1⟩, ⋯⟩
⊢ (v ∘ ⇑(⟨a, b⟩.fst.embedding ((⟨a, b⟩.snd.embedding ⟨i, hi1⟩) ⟨j, hj1⟩))) ⟨k, hk1⟩ =
    ((v ∘ ⇑(⟨a.gather b, a.sigmaCompositionAux b⟩.fst.embedding ⟨i, hi2⟩)) ∘
        ⇑((⟨a.gather b, a.sigmaCompositionAux b⟩.snd ⟨i, hi2⟩).embedding ⟨j, hj2⟩))
      ⟨k, hk2⟩

Tactic:
refine congr_arg v (Fin.ext ?_)

Result:
case h.H.mk
𝕜 : Type u_1
E : Type u_2
F : Type u_3
G : Type u_4
H : Type u_5
inst✝⁸ : NontriviallyNormedField 𝕜
inst✝⁷ : NormedAddCommGroup E
inst✝⁶ : NormedSpace 𝕜 E
inst✝⁵ : NormedAddCommGroup F
inst✝⁴ : NormedSpace 𝕜 F
inst✝³ : NormedAddCommGroup G
inst✝² : NormedSpace 𝕜 G
inst✝¹ : NormedAddCommGroup H
inst✝ : NormedSpace 𝕜 H
r : FormalMultilinearSeries 𝕜 G H
q : FormalMultilinearSeries 𝕜 F G
p : FormalMultilinearSeries 𝕜 E F
n : ℕ
v : Fin n → E
f : (a : Composition n) × Composition a.length → H :=
  fun c => (r c.snd.length) (q.applyComposition c.snd (p.applyComposition c.fst v))
g : (c : Composition n) × ((i : Fin c.length) → Composition (c.blocksFun i)) → H :=
  fun c => (r c.fst.length) fun i => (q (c.snd i).length) (p.applyComposition (c.snd i) (v ∘ ⇑(c.fst.embedding i)))
a : Composition n
b : Composition a.length
a✝ : ⟨a, b⟩ ∈ Finset.univ
i : ℕ
hi1 : i < b.length
hi2 : i < (a.gather b).length
j : ℕ
hj1 : j < b.blocksFun ⟨i, hi1⟩
hj2 : j < (a.sigmaCompositionAux b ⟨↑⟨i, hi1⟩, ⋯⟩).length
k : ℕ
hk1 : k < a.blocksFun ((b.embedding ⟨i, hi1⟩) ⟨j, hj1⟩)
hk2 : k < (a.sigmaCompositionAux b ⟨↑⟨i, hi1⟩, ⋯⟩).blocksFun ⟨↑⟨j, hj1⟩, ⋯⟩
⊢ ↑((⟨a, b⟩.fst.embedding ((⟨a, b⟩.snd.embedding ⟨i, hi1⟩) ⟨j, hj1⟩)) ⟨k, hk1⟩) =
    ↑((⟨a.gather b, a.sigmaCompositionAux b⟩.fst.embedding ⟨i, hi2⟩)
        (((⟨a.gather b, a.sigmaCompositionAux b⟩.snd ⟨i, hi2⟩).embedding ⟨j, hj2⟩) ⟨k, hk2⟩))