∀ a, b. a ∈ x0 → a = b ∨ a ∈ x0 -1l b, a ∈ a0 + x0 ⊦ a = b, a ∈ (a0 + x0) -1l b
Used simplifier rules: list freely generated by [], +;;%in-empty: ⊦ ¬ a ∈ []; from specification list