| n ≤ #(a + x0), ∀ n. n ≤ # x0 → firstn(n, x0) ⊑ x0 ⊦ firstn(n, a + x0) ⊑ a + x0 |
n ≤ 1 + # x0, ∀ n. n ≤ # x0 → firstn(n, x0) ⊑ x0 ⊦ firstn(n, a ' + x0) ⊑ a ' + x0
Used simplifier rules: a: ⊦ (x + y) + z = x + y + z; from specification list% ⊦ # a ' = 0 +1; (s)% ⊦ #(x + y) = # x + # y; (s)% ⊦ a + x = a ' + x; (s)