Used simplifier rules: in-10: ⊦ ¬ a ∈ x → (a ∈ b ' + x ↔ a = b);%a: ⊦ (x + y) + z = x + y + z;%in-06: ⊦ a ∈ b ' + x ↔ ¬(a ≠ b ∧ ¬ a ∈ x);