-- Mathlib/Data/Matroid/Sum.lean
@[simp] lemma sigma_isBasis_iff {I X} :
    (Matroid.sigma M).IsBasis I X ↔ ∀ i, (M i).IsBasis (Sigma.mk i ⁻¹' I) (Sigma.mk i ⁻¹' X)

Proof state:
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

Tactic:
refine hII₀.antisymm ?_

Result:
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
⊢ I₀ ⊆ Sigma.mk i ⁻¹' I

Proof state:
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''⟩ ↦ ⟨?_, ?_, ?_⟩⟩

Result:
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

Proof state:
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' : 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
h : (∀ (i_1 : ι), (M i_1).Indep (Sigma.mk i_1 ⁻¹' I ∪ Sigma.mk i_1 ⁻¹' (Sigma.mk i '' I₀))) → I = I ∪ Sigma.mk i '' I₀
j : ι
⊢ (M j).Indep (Sigma.mk j ⁻¹' I ∪ Sigma.mk j ⁻¹' (Sigma.mk i '' I₀))

Tactic:
obtain (rfl | hij) := eq_or_ne i j

Result:
case refine_1.inl
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' : 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
h : (∀ (i_1 : ι), (M i_1).Indep (Sigma.mk i_1 ⁻¹' I ∪ Sigma.mk i_1 ⁻¹' (Sigma.mk i '' I₀))) → I = I ∪ Sigma.mk i '' I₀
⊢ (M i).Indep (Sigma.mk i ⁻¹' I ∪ Sigma.mk i ⁻¹' (Sigma.mk i '' I₀))

case refine_1.inr
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' : 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
h : (∀ (i_1 : ι), (M i_1).Indep (Sigma.mk i_1 ⁻¹' I ∪ Sigma.mk i_1 ⁻¹' (Sigma.mk i '' I₀))) → I = I ∪ Sigma.mk i '' I₀
j : ι
hij : i ≠ j
⊢ (M j).Indep (Sigma.mk j ⁻¹' I ∪ Sigma.mk j ⁻¹' (Sigma.mk i '' I₀))

Proof state:
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

Tactic:
exact fun ⟨i,x⟩ hx ↦ by simpa using h'' i hx

Result:
no goals