Used simplifier rules: a: ⊦ (x + y) + z = x + y + z;%inapp-03: ⊦ ¬ a ∈ y ∧ ¬ b ∈ x → (x + a ' + z = y + b ' + y0 ↔ a = b ∧ x = y ∧ z = y0);