ordered≤(x0), a + x1 ⊑ x0, ∀ x0. ordered≤(x0) ∧ x1 ⊑ x0 → ordered≤(x1) ⊦ ordered≤(a + x1)
Used simplifier rules: le-e: ⊦ ordered≤([]);%list freely generated by [], +;;% ⊦ [] ⊑ x; (s)