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