x0 + y = x0 + z, ∀ y, z. x0 + y = x0 + z → y = z ⊦ y = z
Used simplifier rules: Cons: ⊦ (a + x) + y = a + (x + y);% ⊦ a + x = a0 + x0 ↔ a = a0 ∧ x = x0; (ws)