∀ y, z. (x0 + y) + z = x0 + y + z ⊦ (x0 + y) + z = x0 + y + z
Used simplifier rules: Cons: ⊦ (a + x) + y = a + (x + y);% ⊦ a + x = a + y ↔ x = y; (s)