Used simplifier rules: in-rec: ⊦ a ∈ b ' + x ↔ ¬ ¬(a = b ∨ a ∈ x);%a: ⊦ (x + y) + z = x + y + z;%in-01: ⊦ a ≠ b → (a ∈ b ' + x ↔ a ∈ x);