x0 + y ⊑ x0, ∀ y. x0 + y ⊑ x0 → y = [] ⊦ y = []
Used simplifier rules: postifx-app-03: ⊦ x + y ⊑ x + z ↔ y ⊑ z;%a: ⊦ (x + y) + z = x + y + z; from specification list% ⊦ a + x = a ' + x; (s)