i1 * j + j1 = i0 * j + j0, j0 + j < 0, j < 0, j1 + j < 0, i0 ≤ 0, 0 ≤ j0, 0 ≤ j1, 0 ≤ i0 * j + j0 ⊦ i0 = i1 ∧ j0 = j1