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