-- Lean 4 proof step from Mathlib.ModelTheory.ElementaryMaps (goal)
Goal:
L : Language
M : Type u_1
N : Type u_2
inst : L.Structure M
inst_1 : L.Structure N
f : M ↪ₑ[L] N
α : Type u_5
n : ℕ
φ : L.BoundedFormula α n
v : α → M
xs : Fin n → M
rw :
  BoundedFormula.Realize.{u_6, u_7, u_2, u_5} (M := N) (α := (↑(↑φ.freeVarFinset : Set α) : Type u_5))
      (φ.restrictFreeVar (Set.inclusion (s := (↑φ.freeVarFinset : Set α)) (t := (↑φ.freeVarFinset : Set α)) ⋯))
      (((⇑f : (a : M) → N) ∘ v) ∘ Subtype.val (p := fun (x : α) ↦ x ∈ (↑φ.freeVarFinset : Set α)))
      ((⇑f : (a : M) → N) ∘ xs) ↔
    φ.Realize v xs
rw₁ rw₂ rw₃ :
  BoundedFormula.Realize.{u_6, u_7, u_2, u_5} (M := N) (α := (↑(↑φ.freeVarFinset : Set α) : Type u_5))
      (φ.restrictFreeVar id)
      (((⇑f : (a : M) → N) ∘ v) ∘ Subtype.val (p := fun (x : α) ↦ x ∈ (↑φ.freeVarFinset : Set α)))
      ((⇑f : (a : M) → N) ∘ xs) ↔
    φ.Realize v xs
⊢ BoundedFormula.Realize.{u_6, u_7, u_2, u_5} (M := N) φ ((⇑f : (a : M) → N) ∘ v) ((⇑f : (a : M) → N) ∘ xs) ↔
    φ.Realize v xs
have rw₄ : ∀
  (h :
    Formula.Realize (α := Fin (Fintype.card.{u_5} (↥φ.freeVarFinset ⊕ Fin n)))
        (Formula.relabel (α := ↥φ.freeVarFinset ⊕ Fin n)
          (⇑(Fintype.equivFin (↥φ.freeVarFinset ⊕ Fin n)) :
            (a : ↥φ.freeVarFinset ⊕ Fin n) → Fin (Fintype.card (↥φ.freeVarFinset ⊕ Fin n)))
          (BoundedFormula.toFormula (φ.restrictFreeVar id)))
        ((↑f : M → N) ∘
          Function.comp (β := ↥φ.freeVarFinset ⊕ Fin n)
            (Sum.elim (v ∘ Subtype.val (p := fun (x : α) ↦ x ∈ φ.freeVarFinset)) xs)
            (⇑(_root_.Equiv.symm (Fintype.equivFin (↥φ.freeVarFinset ⊕ Fin n))) :
              (a : Fin (Fintype.card (↥φ.freeVarFinset ⊕ Fin n))) → ↥φ.freeVarFinset ⊕ Fin n)) ↔
      Formula.Realize (α := Fin (Fintype.card.{u_5} (↥φ.freeVarFinset ⊕ Fin n)))
        (Formula.relabel (α := ↥φ.freeVarFinset ⊕ Fin n)
          (⇑(Fintype.equivFin (↥φ.freeVarFinset ⊕ Fin n)) :
            (a : ↥φ.freeVarFinset ⊕ Fin n) → Fin (Fintype.card (↥φ.freeVarFinset ⊕ Fin n)))
          (BoundedFormula.toFormula (φ.restrictFreeVar id)))
        (Function.comp (β := ↥φ.freeVarFinset ⊕ Fin n)
          (Sum.elim (v ∘ Subtype.val (p := fun (x : α) ↦ x ∈ φ.freeVarFinset)) xs)
          (⇑(_root_.Equiv.symm (Fintype.equivFin (↥φ.freeVarFinset ⊕ Fin n))) :
            (a : Fin (Fintype.card (↥φ.freeVarFinset ⊕ Fin n))) → ↥φ.freeVarFinset ⊕ Fin n))),
  BoundedFormula.Realize.{u_6, u_7, u_2, u_5} (M := N) (α := (↑(↑φ.freeVarFinset : Set α) : Type u_5))
      (φ.restrictFreeVar id)
      (((⇑f : (a : M) → N) ∘ v) ∘ Subtype.val (p := fun (x : α) ↦ x ∈ (↑φ.freeVarFinset : Set α)))
      ((⇑f : (a : M) → N) ∘ xs) ↔
    φ.Realize v xs
Tactic: classical
    rw [← BoundedFormula.realize_restrictFreeVar' Set.Subset.rfl, Set.inclusion_eq_id]
    have h :=
      f.map_formula' ((φ.restrictFreeVar id).toFormula.relabel (Fintype.equivFin _))
        (Sum.elim (v ∘ (↑)) xs ∘ (Fintype.equivFin _).symm)
    simp only [Formula.realize_relabel, BoundedFormula.realize_toFormula] at h
    rw [← Function.comp_assoc _ _ (Fintype.equivFin _).symm,
      Function.comp_assoc _ (Fintype.equivFin _).symm (Fintype.equivFin _),
      _root_.Equiv.symm_comp_self, Function.comp_id, Function.comp_assoc, Sum.elim_comp_inl,
      Function.comp_assoc _ _ Sum.inr, Sum.elim_comp_inr, ← Function.comp_assoc] at h
    refine h.trans ?_
    rw [Function.comp_assoc _ _ (Fintype.equivFin _), _root_.Equiv.symm_comp_self,
      Function.comp_id, Sum.elim_comp_inl, Sum.elim_comp_inr (v ∘ Subtype.val) xs,
      BoundedFormula.realize_restrictFreeVar v (by simp)]