disj(x, y), a ∈ y, disj(a ' + x, y) ⊦
Used simplifier rules: first-01: ⊦ ¬ a ∈ x → (disj(a ' + y, x) ↔ disj(y, x));%a: ⊦ (x + y) + z = x + y + z; from specification list