| n ≤ 1 + # x0, ∀ n. n ≤ # x0 → firstn(n, firstn(n, x0)) = firstn(n, x0) ⊦ firstn(n, a ' + firstn(n -1, x0)) = a ' + firstn(n -1, x0), n = 0 |
n ≤ 1 + # x0, ∀ n. n ≤ # x0 → firstn(n, firstn(n, x0)) = firstn(n, x0) ⊦ firstn(n, a ' + firstn(n - 1, x0)) = a ' + firstn(n - 1, x0), n = 0
Used simplifier rules: ⊦ n -1 = n - 1; (s)