-- 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 : ℕ
c_1 c : OrderedFinpartition n
j i : Fin c.length
h :
  (∀ (hij : i.castSucc < j.castSucc),
      LT.lt (α := Fin (n + 1))
        ((fun (m : Fin (c.length + 1)) ↦
            (cases (motive := fun (x : Fin (c.length + 1)) ↦
                Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize x) → Fin (n + 1))
                (fun (x : Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize 0)) ↦ 0)
                (fun (m : Fin c.length) ↦ succ ∘ c.emb m) m
                ⟨HSub.hSub (α := ℕ) (β := ℕ) (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize m : ℕ) 1, ⋯⟩ :
              Fin (n + 1)))
          i.castSucc)
        ((fun (m : Fin (c.length + 1)) ↦
            (cases (motive := fun (x : Fin (c.length + 1)) ↦
                Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize x) → Fin (n + 1))
                (fun (x : Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize 0)) ↦ 0)
                (fun (m : Fin c.length) ↦ succ ∘ c.emb m) m
                ⟨HSub.hSub (α := ℕ) (β := ℕ) (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize m : ℕ) 1, ⋯⟩ :
              Fin (n + 1)))
          j.castSucc)) →
    ∀ (hij : i.castSucc < j.succ),
      LT.lt (α := Fin (n + 1))
        ((fun (m : Fin (c.length + 1)) ↦
            (cases (motive := fun (x : Fin (c.length + 1)) ↦
                Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize x) → Fin (n + 1))
                (fun (x : Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize 0)) ↦ 0)
                (fun (m : Fin c.length) ↦ succ ∘ c.emb m) m
                ⟨HSub.hSub (α := ℕ) (β := ℕ) (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize m : ℕ) 1, ⋯⟩ :
              Fin (n + 1)))
          i.castSucc)
        ((fun (m : Fin (c.length + 1)) ↦
            (cases (motive := fun (x : Fin (c.length + 1)) ↦
                Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize x) → Fin (n + 1))
                (fun (x : Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize 0)) ↦ 0)
                (fun (m : Fin c.length) ↦ succ ∘ c.emb m) m
                ⟨HSub.hSub (α := ℕ) (β := ℕ) (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize m : ℕ) 1, ⋯⟩ :
              Fin (n + 1)))
          j.succ)
h_1 :
  ∀ (hij : i.succ < j.castSucc),
    LT.lt (α := Fin (n + 1))
      ((fun (m : Fin (c.length + 1)) ↦
          (cases (motive := fun (x : Fin (c.length + 1)) ↦
              Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize x) → Fin (n + 1))
              (fun (x : Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize 0)) ↦ 0)
              (fun (m : Fin c.length) ↦ succ ∘ c.emb m) m
              ⟨HSub.hSub (α := ℕ) (β := ℕ) (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize m : ℕ) 1, ⋯⟩ :
            Fin (n + 1)))
        i.succ)
      ((fun (m : Fin (c.length + 1)) ↦
          (cases (motive := fun (x : Fin (c.length + 1)) ↦
              Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize x) → Fin (n + 1))
              (fun (x : Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize 0)) ↦ 0)
              (fun (m : Fin c.length) ↦ succ ∘ c.emb m) m
              ⟨HSub.hSub (α := ℕ) (β := ℕ) (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize m : ℕ) 1, ⋯⟩ :
            Fin (n + 1)))
        j.castSucc)
hij : i.succ < j.succ
⊢ LT.lt (α := Fin (n + 1))
    ((fun (m : Fin (c.length + 1)) ↦
        (cases (motive := fun (x : Fin (c.length + 1)) ↦
            Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize x) → Fin (n + 1))
            (fun (x : Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize 0)) ↦ 0)
            (fun (m : Fin c.length) ↦ succ ∘ c.emb m) m
            ⟨HSub.hSub (α := ℕ) (β := ℕ) (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize m : ℕ) 1, ⋯⟩ :
          Fin (n + 1)))
      i.succ)
    ((fun (m : Fin (c.length + 1)) ↦
        (cases (motive := fun (x : Fin (c.length + 1)) ↦
            Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize x) → Fin (n + 1))
            (fun (x : Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize 0)) ↦ 0)
            (fun (m : Fin c.length) ↦ succ ∘ c.emb m) m
            ⟨HSub.hSub (α := ℕ) (β := ℕ) (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize m : ℕ) 1, ⋯⟩ :
          Fin (n + 1)))
      j.succ)
have succ : c.emb i (⟨c.partSize i - 1, sorry⟩ : Fin (cons (α := fun (a : Fin (c.length + 1)) ↦ ℕ) 1 c.partSize i.succ)) <
  c.emb j ⟨c.partSize j - 1, sorry⟩
Tactic: simp only [cons_succ, cases_succ, comp_apply, succ_lt_succ_iff]