| n ≤ # x0, ∀ n. n ≤ # x0 → butlastn(n, x0) = firstn(# x0 - n, x0) ⊦ 1 + # x0 = n, butlastn(n, a ' + x0) = a ' + firstn(# x0 - n, x0) |
⊦ n ≤ # x → butlastn(n, a ' + x) = a ' + butlastn(n, x)
n ≤ # x0, ∀ n. n ≤ # x0 → butlastn(n, x0) = firstn(# x0 - n, x0) ⊦ a ' + butlastn(n, x0) = a ' + firstn(# x0 - n, x0), 1 + # x0 = n
Used terms: x0, a, n