theorem boundedFormula_realize_cast {β : Type*} {n : ℕ} (φ : L.BoundedFormula β n)
    (x : β → ∀ a, M a) (v : Fin n → ∀ a, M a) :
    (φ.Realize (fun i : β => (x i : (u : Filter α).Product M))
        (fun i => (v i : (u : Filter α).Product M))) ↔
      ∀ᶠ a : α in u, φ.Realize (fun i : β => x i a) fun i => v i a := by
  letI := (u : Filter α).productSetoid M
  induction' φ with _ _ _ _ _ _ _ _ m _ _ ih ih' k φ ih
  · simp only [BoundedFormula.Realize, eventually_const]
  · have h2 : ∀ a : α, (Sum.elim (fun i : β => x i a) fun i => v i a) = fun i => Sum.elim x v i a :=
      fun a => funext fun i => Sum.casesOn i (fun i => rfl) fun i => rfl
    simp only [BoundedFormula.Realize, h2, term_realize_cast]
    erw [(Sum.comp_elim ((↑) : (∀ a, M a) → (u : Filter α).Product M) x v).symm,
      term_realize_cast, term_realize_cast]
    exact Quotient.eq''
  · have h2 : ∀ a : α, (Sum.elim (fun i : β => x i a) fun i => v i a) = fun i => Sum.elim x v i a :=
      fun a => funext fun i => Sum.casesOn i (fun i => rfl) fun i => rfl
    simp only [BoundedFormula.Realize, h2]
    erw [(Sum.comp_elim ((↑) : (∀ a, M a) → (u : Filter α).Product M) x v).symm]
    conv_lhs => enter [2, i]; erw [term_realize_cast]
    apply relMap_quotient_mk'
  · simp only [BoundedFormula.Realize, ih v, ih' v]
    rw [Ultrafilter.eventually_imp]
  · simp only [BoundedFormula.Realize]
    apply Iff.trans (b := ∀ m : ∀ a : α, M a,
      φ.Realize (fun i : β => (x i : (u : Filter α).Product M))
        (Fin.snoc (((↑) : (∀ a, M a) → (u : Filter α).Product M) ∘ v)
          (m : (u : Filter α).Product M)))
    · exact Quotient.forall
    have h' :
      ∀ (m : ∀ a, M a) (a : α),
        (fun i : Fin (k + 1) => (Fin.snoc v m : _ → ∀ a, M a) i a) =
          Fin.snoc (fun i : Fin k => v i a) (m a) := by
      refine' fun m a => funext (Fin.reverseInduction _ fun i _ => _)
      · simp only [Fin.snoc_last]
      · simp only [Fin.snoc_castSucc]
    simp only [← Fin.comp_snoc]
    simp only [Function.comp, ih, h']
    refine' ⟨fun h => _, fun h m => _⟩
    · contrapose! h
      simp_rw [← Ultrafilter.eventually_not, not_forall] at h
      refine'
        ⟨fun a : α =>
          Classical.epsilon fun m : M a =>
            ¬φ.Realize (fun i => x i a) (Fin.snoc (fun i => v i a) m),
          _⟩
      rw [← Ultrafilter.eventually_not]
      exact Filter.mem_of_superset h fun a ha => Classical.epsilon_spec ha
    · rw [Filter.eventually_iff] at *
      

Proof state:
case all.h₂.refine'_2
α : Type u_1
M : α → Type u_2
u : Ultrafilter α
L : Language
inst✝¹ : (a : α) → Structure L (M a)
inst✝ : ∀ (a : α), Nonempty (M a)
β : Type u_3
n : ℕ
x : β → (a : α) → M a
this : Setoid ((a : α) → M a) := productSetoid (↑u) M
k : ℕ
φ : BoundedFormula L β (k + 1)
ih :
  ∀ (v : Fin (k + 1) → (a : α) → M a),
    (BoundedFormula.Realize φ (fun i => Quotient.mk' (x i)) fun i => Quotient.mk' (v i)) ↔
      ∀ᶠ (a : α) in ↑u, BoundedFormula.Realize φ (fun i => x i a) fun i => v i a
v : Fin k → (a : α) → M a
h' : ∀ (m : (a : α) → M a) (a : α), (fun i => Fin.snoc v m i a) = Fin.snoc (fun i => v i a) (m a)
h : {x_1 | ∀ (x_2 : M x_1), BoundedFormula.Realize φ (fun i => x i x_1) (Fin.snoc (fun i => v i x_1) x_2)} ∈ ↑u
m : (a : α) → M a
⊢ {x_1 | BoundedFormula.Realize φ (fun i => x i x_1) (Fin.snoc (fun i => v i x_1) (m x_1))} ∈ ↑u

Next tactic:
exact Filter.mem_of_superset h fun a ha => ha (m a)