∀ a. ¬(a ∈ x0 ∧ # x0 ≤ pos(a, x0)), a ∈ a0 + x0, #(a0 + x0) ≤ pos(a, a0 + x0) ⊦
Used simplifier rules: list freely generated by [], +;;%in-empty: ⊦ ¬ a ∈ []; from specification list