-- Lean 4 proof step from Mathlib.Analysis.Calculus.ContDiff.FaaDiBruno (goal)
Goal:
𝕜 : Type u_1
inst : NontriviallyNormedField 𝕜
E : Type u_2
inst_1 : NormedAddCommGroup E
inst_2 : NormedSpace 𝕜 E
F : Type u_3
inst_3 : NormedAddCommGroup F
inst_4 : NormedSpace 𝕜 F
G : Type u_4
inst_5 : NormedAddCommGroup G
inst_6 : NormedSpace 𝕜 G
s : Set E
t : Set F
q : F → FormalMultilinearSeries 𝕜 F G
p : E → FormalMultilinearSeries 𝕜 E F
n_1 : ℕ
c_1 : OrderedFinpartition n_1
n : ℕ
c : OrderedFinpartition (n + 1)
h : range (ι := Fin (c.partSize 0)) (c.emb 0) = {0}
A : c.length - 1 + 1 = c.length
pos :
  length (n := n + 1)
      { length := c.length - 1 + 1,
        partSize :=
          cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1 fun (i : Fin (c.length - 1)) ↦
            c.partSize (Fin.cast ⋯ i.succ),
        partSize_pos := ⋯,
        emb := fun (i : Fin (c.length - 1 + 1)) ↦
          cases (motive := fun (x : Fin (c.length - 1 + 1)) ↦
            Fin
                (cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1
                  (fun (i : Fin (c.length - 1)) ↦ c.partSize (Fin.cast ⋯ i.succ)) x) →
              Fin (n + 1))
            (fun
                (x :
                  Fin
                    (cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1
                      (fun (i : Fin (c.length - 1)) ↦ c.partSize (Fin.cast ⋯ i.succ)) 0)) ↦
              0)
            (fun (m : Fin (c.length - 1)) ↦
              succ ∘ fun (j : Fin (c.partSize (Fin.cast ⋯ m.succ))) ↦ (c.emb (Fin.cast ⋯ m.succ) j).pred ⋯)
            i,
        emb_strictMono := ⋯, parts_strictMono := ⋯, disjoint := ⋯, cover := ⋯ : OrderedFinpartition (n + 1) } =
    c.length
⊢ { length := c.length - 1 + 1,
      partSize :=
        cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1 fun (i : Fin (c.length - 1)) ↦
          c.partSize (Fin.cast ⋯ i.succ),
      partSize_pos := ⋯,
      emb := fun (i : Fin (c.length - 1 + 1)) ↦
        cases (motive := fun (x : Fin (c.length - 1 + 1)) ↦
          Fin
              (cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1
                (fun (i : Fin (c.length - 1)) ↦ c.partSize (Fin.cast ⋯ i.succ)) x) →
            Fin (n + 1))
          (fun
              (x :
                Fin
                  (cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1
                    (fun (i : Fin (c.length - 1)) ↦ c.partSize (Fin.cast ⋯ i.succ)) 0)) ↦
            0)
          (fun (m : Fin (c.length - 1)) ↦
            succ ∘ fun (j : Fin (c.partSize (Fin.cast ⋯ m.succ))) ↦ (c.emb (Fin.cast ⋯ m.succ) j).pred ⋯)
          i,
      emb_strictMono := ⋯, parts_strictMono := ⋯, disjoint := ⋯, cover := ⋯ } =
    c
have pos₁ : HEq (α :=
  Fin
      (length (n := n + 1)
        { length := c.length - 1 + 1,
          partSize :=
            cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1 fun (i : Fin (c.length - 1)) ↦
              c.partSize (Fin.cast sorry i.succ),
          partSize_pos := sorry,
          emb := fun (i : Fin (c.length - 1 + 1)) ↦
            cases (motive := fun (x : Fin (c.length - 1 + 1)) ↦
              Fin
                  (cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1
                    (fun (i : Fin (c.length - 1)) ↦ c.partSize (Fin.cast sorry i.succ)) x) →
                Fin (n + 1))
              (fun
                  (x :
                    Fin
                      (cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1
                        (fun (i : Fin (c.length - 1)) ↦ c.partSize (Fin.cast sorry i.succ)) 0)) ↦
                0)
              (fun (m : Fin (c.length - 1)) ↦
                succ ∘ fun (j : Fin (c.partSize (Fin.cast sorry m.succ))) ↦ (c.emb (Fin.cast sorry m.succ) j).pred sorry)
              i,
          emb_strictMono := sorry, parts_strictMono := sorry, disjoint := sorry, cover := sorry }) →
    ℕ)
  (partSize
    { length := c.length - 1 + 1,
      partSize :=
        cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1 fun (i : Fin (c.length - 1)) ↦
          c.partSize (Fin.cast sorry i.succ),
      partSize_pos := sorry,
      emb := fun (i : Fin (c.length - 1 + 1)) ↦
        cases (motive := fun (x : Fin (c.length - 1 + 1)) ↦
          Fin
              (cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1
                (fun (i : Fin (c.length - 1)) ↦ c.partSize (Fin.cast sorry i.succ)) x) →
            Fin (n + 1))
          (fun
              (x :
                Fin
                  (cons (α := fun (a : Fin (c.length - 1 + 1)) ↦ ℕ) 1
                    (fun (i : Fin (c.length - 1)) ↦ c.partSize (Fin.cast sorry i.succ)) 0)) ↦
            0)
          (fun (m : Fin (c.length - 1)) ↦
            succ ∘ fun (j : Fin (c.partSize (Fin.cast sorry m.succ))) ↦ (c.emb (Fin.cast sorry m.succ) j).pred sorry)
          i,
      emb_strictMono := sorry, parts_strictMono := sorry, disjoint := sorry, cover := sorry })
  c.partSize
Tactic: ext