0 ≤ i, 0 ≤ j ⊦ fac(i + j) = fac(i) * Π countFun(1, j, λ j0. i + j0)
Used simplifier rules: forward-le-01: ⊦ i ≤ j ∧ 0 ≤ i → 0 ≤ j; (forward) from specification int-abs