ordered≤(y), ordered≤(a + x0), perm(a + x0, y), ∀ y. ordered≤(x0) ∧ ordered≤(y) ∧ perm(x0, y) → x0 = y ⊦ a + x0 = y
Used simplifier rules: list freely generated by [], +;;% ⊦ perm([], y) ↔ y = []; (s)