disj(x, y), a ∈ y, ∀ a0. ¬ a0 ∈ x ∧ a0 ≠ a ∨ ¬ a0 ∈ y ⊦
Used simplifier rules: a: ⊦ (x + y) + z = x + y + z; from specification list% ⊦ a ∈ b ' ↔ a = b; (s)% ⊦ a ∈ x + y ↔ a ∈ x ∨ a ∈ y; (s)