Theorem: SetTheory.union_assoc (SetTheory/Basic.lean)

State before:
V : Type u_1
inst✝ : Zermelo V
x y z : V
⊢ x ∪ y ∪ z = x ∪ (y ∪ z)
Tactic:
ext
State after:
case h
V : Type u_1
inst✝ : Zermelo V
x y z z✝ : V
⊢ z✝ ∈ x ∪ y ∪ z ↔ z✝ ∈ x ∪ (y ∪ z)

State before:
case h
V : Type u_1
inst✝ : Zermelo V
x y z z✝ : V
⊢ z✝ ∈ x ∪ y ∪ z ↔ z✝ ∈ x ∪ (y ∪ z)
Tactic:
aesop
State after:
no goals