i ≤ j, i ≤ 0, j ≤ 0 ⊦ i→n(i - j) = i→n(i) - i→n(j)
Used simplifier rules: forward-le-04: ⊦ j ≤ i ∧ i ≤ 0 → j ≤ 0; (forward) from specification int-abs