intro l _ h
case mpr.mk
β : Type u_2
inst✝ : DecidableEq β
α : Type u_1
e : β
s : Finset α
f : α → Finset β
x : α
h₂ : e ∈ f x
⊢ ∀ (a : List α) (nodup : Multiset.Nodup (Quot.mk Setoid.r a)),
    x ∈ { val := Quot.mk Setoid.r a, nodup := nodup } →
      e ∈ fold Union.union ∅ f { val := Quot.mk Setoid.r a, nodup := nodup }
case mpr.mk
β : Type u_2
inst✝ : DecidableEq β
α : Type u_1
e : β
s : Finset α
f : α → Finset β
x : α
h₂ : e ∈ f x
l : List α
nodup✝ : Multiset.Nodup (Quot.mk Setoid.r l)
h : x ∈ { val := Quot.mk Setoid.r l, nodup := nodup✝ }
⊢ e ∈ fold Union.union ∅ f { val := Quot.mk Setoid.r l, nodup := nodup✝ }