| perm(x, y), ordered≤(x), ordered≤(y) ⊦ x = y |
∀ y. perm(x0, y) ∧ ordered≤(x0) ∧ ordered≤(y) → x0 = y, perm(a + x0, y), ordered≤(a + x0), ordered≤(y) ⊦ a + x0 = y
Used simplifier rules: list freely generated by [], +;;%perm-r: ⊦ y ≠ [] → ¬ perm([], y); from specification list-perm