Module: Mathlib.Combinatorics.Matroid.Sum

Goal state before:
pp: ι : Type u_1
α : ι → Type u_2
M : (i : ι) → Matroid (α i)
I X : Set ((i : ι) × α i)
⊢ (∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' I)) →
    ((I ⊆ X ∧
        (∀ ⦃t : Set ((i : ι) × α i)⦄, (∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' t)) → t ⊆ X → I ⊆ t → I = t) ∧
          X ⊆ univ.sigma fun i ↦ (M i).E) ↔
      (∀ (x : ι), Sigma.mk x ⁻¹' I ⊆ Sigma.mk x ⁻¹' X) ∧
        (∀ (x : ι) ⦃t : Set (α x)⦄,
            (M x).Indep t → t ⊆ Sigma.mk x ⁻¹' X → Sigma.mk x ⁻¹' I ⊆ t → Sigma.mk x ⁻¹' I = t) ∧
          ∀ (x : ι), Sigma.mk x ⁻¹' X ⊆ (M x).E)

Tactic:
refine fun hI ↦ ⟨fun ⟨hIX, h, h'⟩ ↦ ⟨fun i ↦ preimage_mono hIX, fun i I₀ hI₀ hI₀X hII₀ ↦ ?_, ?_⟩,
    fun ⟨hIX, h', h''⟩ ↦ ⟨?_, ?_, ?_⟩⟩

Goal state after:
case refine_1
ι : Type u_1
α : ι → Type u_2
M : (i : ι) → Matroid (α i)
I X : Set ((i : ι) × α i)
hI : ∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' I)
x✝ :
  I ⊆ X ∧
    (∀ ⦃t : Set ((i : ι) × α i)⦄, (∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' t)) → t ⊆ X → I ⊆ t → I = t) ∧
      X ⊆ univ.sigma fun i ↦ (M i).E
hIX : I ⊆ X
h : ∀ ⦃t : Set ((i : ι) × α i)⦄, (∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' t)) → t ⊆ X → I ⊆ t → I = t
h' : X ⊆ univ.sigma fun i ↦ (M i).E
i : ι
I₀ : Set (α i)
hI₀ : (M i).Indep I₀
hI₀X : I₀ ⊆ Sigma.mk i ⁻¹' X
hII₀ : Sigma.mk i ⁻¹' I ⊆ I₀
⊢ Sigma.mk i ⁻¹' I = I₀

case refine_2
ι : Type u_1
α : ι → Type u_2
M : (i : ι) → Matroid (α i)
I X : Set ((i : ι) × α i)
hI : ∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' I)
x✝ :
  I ⊆ X ∧
    (∀ ⦃t : Set ((i : ι) × α i)⦄, (∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' t)) → t ⊆ X → I ⊆ t → I = t) ∧
      X ⊆ univ.sigma fun i ↦ (M i).E
hIX : I ⊆ X
h : ∀ ⦃t : Set ((i : ι) × α i)⦄, (∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' t)) → t ⊆ X → I ⊆ t → I = t
h' : X ⊆ univ.sigma fun i ↦ (M i).E
⊢ ∀ (x : ι), Sigma.mk x ⁻¹' X ⊆ (M x).E

case refine_3
ι : Type u_1
α : ι → Type u_2
M : (i : ι) → Matroid (α i)
I X : Set ((i : ι) × α i)
hI : ∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' I)
x✝ :
  (∀ (x : ι), Sigma.mk x ⁻¹' I ⊆ Sigma.mk x ⁻¹' X) ∧
    (∀ (x : ι) ⦃t : Set (α x)⦄, (M x).Indep t → t ⊆ Sigma.mk x ⁻¹' X → Sigma.mk x ⁻¹' I ⊆ t → Sigma.mk x ⁻¹' I = t) ∧
      ∀ (x : ι), Sigma.mk x ⁻¹' X ⊆ (M x).E
hIX : ∀ (x : ι), Sigma.mk x ⁻¹' I ⊆ Sigma.mk x ⁻¹' X
h' : ∀ (x : ι) ⦃t : Set (α x)⦄, (M x).Indep t → t ⊆ Sigma.mk x ⁻¹' X → Sigma.mk x ⁻¹' I ⊆ t → Sigma.mk x ⁻¹' I = t
h'' : ∀ (x : ι), Sigma.mk x ⁻¹' X ⊆ (M x).E
⊢ I ⊆ X

case refine_4
ι : Type u_1
α : ι → Type u_2
M : (i : ι) → Matroid (α i)
I X : Set ((i : ι) × α i)
hI : ∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' I)
x✝ :
  (∀ (x : ι), Sigma.mk x ⁻¹' I ⊆ Sigma.mk x ⁻¹' X) ∧
    (∀ (x : ι) ⦃t : Set (α x)⦄, (M x).Indep t → t ⊆ Sigma.mk x ⁻¹' X → Sigma.mk x ⁻¹' I ⊆ t → Sigma.mk x ⁻¹' I = t) ∧
      ∀ (x : ι), Sigma.mk x ⁻¹' X ⊆ (M x).E
hIX : ∀ (x : ι), Sigma.mk x ⁻¹' I ⊆ Sigma.mk x ⁻¹' X
h' : ∀ (x : ι) ⦃t : Set (α x)⦄, (M x).Indep t → t ⊆ Sigma.mk x ⁻¹' X → Sigma.mk x ⁻¹' I ⊆ t → Sigma.mk x ⁻¹' I = t
h'' : ∀ (x : ι), Sigma.mk x ⁻¹' X ⊆ (M x).E
⊢ ∀ ⦃t : Set ((i : ι) × α i)⦄, (∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' t)) → t ⊆ X → I ⊆ t → I = t

case refine_5
ι : Type u_1
α : ι → Type u_2
M : (i : ι) → Matroid (α i)
I X : Set ((i : ι) × α i)
hI : ∀ (i : ι), (M i).Indep (Sigma.mk i ⁻¹' I)
x✝ :
  (∀ (x : ι), Sigma.mk x ⁻¹' I ⊆ Sigma.mk x ⁻¹' X) ∧
    (∀ (x : ι) ⦃t : Set (α x)⦄, (M x).Indep t → t ⊆ Sigma.mk x ⁻¹' X → Sigma.mk x ⁻¹' I ⊆ t → Sigma.mk x ⁻¹' I = t) ∧
      ∀ (x : ι), Sigma.mk x ⁻¹' X ⊆ (M x).E
hIX : ∀ (x : ι), Sigma.mk x ⁻¹' I ⊆ Sigma.mk x ⁻¹' X
h' : ∀ (x : ι) ⦃t : Set (α x)⦄, (M x).Indep t → t ⊆ Sigma.mk x ⁻¹' X → Sigma.mk x ⁻¹' I ⊆ t → Sigma.mk x ⁻¹' I = t
h'' : ∀ (x : ι), Sigma.mk x ⁻¹' X ⊆ (M x).E
⊢ X ⊆ univ.sigma fun i ↦ (M i).E