-- Mathlib/Algebra/BigOperators/Finprod.lean
theorem finprod_mem_eq_prod_of_inter_mulSupport_eq (f : α → M) {s : Set α} {t : Finset α}
    (h : s ∩ mulSupport f = t.toSet ∩ mulSupport f) : ∏ᶠ i ∈ s, f i = ∏ i ∈ t, f i :=
  finprod_cond_eq_prod_of_cond_iff _ <| by
    intro x hxf
    rw [← mem_mulSupport] at hxf
    refine ⟨fun hx => ?_, fun hx => ?_⟩
    · refine ((mem_inter_iff x t (mulSupport f)).mp ?_).1
      rw [← Set.ext_iff.mp h x, mem_inter_iff]
      exact ⟨hx, hxf⟩
    · refine ((mem_inter_iff x s (mulSupport f)).mp ?_).1
      rw [Set.ext_iff.mp h x, mem_inter_iff]
      exact ⟨hx, hxf⟩

Proof state:
case refine_2
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ t
⊢ x ∈ s ∩ mulSupport f

Tactic:
rw [Set.ext_iff.mp h x, mem_inter_iff]

Result:
case refine_2
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ t
⊢ x ∈ ↑t ∧ x ∈ mulSupport f

Proof state:
case refine_2
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ t
⊢ x ∈ ↑t ∧ x ∈ mulSupport f

Tactic:
exact ⟨hx, hxf⟩

Result:
no goals

Proof state:
case refine_1
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ s
⊢ x ∈ s ∧ x ∈ mulSupport f

Tactic:
exact ⟨hx, hxf⟩

Result:
no goals

Proof state:
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : f x ≠ 1
⊢ x ∈ s ↔ x ∈ t

Tactic:
rw [← mem_mulSupport] at hxf

Result:
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
⊢ x ∈ s ↔ x ∈ t

Proof state:
case refine_2
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ t
⊢ x ∈ s

Tactic:
refine ((mem_inter_iff x s (mulSupport f)).mp ?_).1

Result:
case refine_2
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ t
⊢ x ∈ s ∩ mulSupport f

Proof state:
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
⊢ ∀ {x : α}, f x ≠ 1 → (x ∈ s ↔ x ∈ t)

Tactic:
intro x hxf

Result:
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : f x ≠ 1
⊢ x ∈ s ↔ x ∈ t

Proof state:
case refine_1
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ s
⊢ x ∈ t

Tactic:
refine ((mem_inter_iff x t (mulSupport f)).mp ?_).1

Result:
case refine_1
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ s
⊢ x ∈ ↑t ∩ mulSupport f

Proof state:
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
⊢ x ∈ s ↔ x ∈ t

Tactic:
refine ⟨fun hx => ?_, fun hx => ?_⟩

Result:
case refine_1
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ s
⊢ x ∈ t

case refine_2
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ t
⊢ x ∈ s

Proof state:
case refine_1
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ s
⊢ x ∈ ↑t ∩ mulSupport f

Tactic:
rw [← Set.ext_iff.mp h x, mem_inter_iff]

Result:
case refine_1
α : Type u_1
M : Type u_5
inst✝ : CommMonoid M
f : α → M
s : Set α
t : Finset α
h : s ∩ mulSupport f = ↑t ∩ mulSupport f
x : α
hxf : x ∈ mulSupport f
hx : x ∈ s
⊢ x ∈ s ∧ x ∈ mulSupport f