rw [mem_russellSet_iff, not_and, not_not] at this
case neg
V : Type u_1
inst✝ : Zermelo V
x✝ y z x : V
h : russellSet x ∈ x
h' this : ¬russellSet x ∈ russellSet x
⊢ False
case neg
V : Type u_1
inst✝ : Zermelo V
x✝ y z x : V
h : russellSet x ∈ x
h' : ¬russellSet x ∈ russellSet x
this : russellSet x ∈ x → russellSet x ∈ russellSet x
⊢ False