0 ≤ i, 0 ≤ j ⊦ (i * (i + 1)) / 2 < (j * (j + 1)) / 2 ↔ i < j
Used simplifier rules: forward-le-01: ⊦ i ≤ j ∧ 0 ≤ i → 0 ≤ j; (forward) from specification int-abs