∀ n, a. n < # x0 → (x0 + a ')[n] = x0[n] ∨ x0 = [], n < #(a0 + x0) ⊦ ((a0 + x0) + a ')[n] = (a0 + x0)[n], a0 + x0 = []
Used simplifier rules: list freely generated by [], +;;