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