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

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

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