| lf(0) = a ', m ≤ n0, ∀ n, a. nextl(n, a) ≠ [], ∀ m. m < n0 → lf(m + 1) = nextl(m, lf(m).last) ∧ lf(m) ≠ [] ⊦ (m = n0 + 1 ⊃ nextl(n0, lf(n0).last);lf(m)) ≠ [] |
(m = n0 + 1 ⊃ nextl(n0, lf(n0).last);lf(m)) = [], lf(0) = a ', m ≤ n0, ∀ n, a. nextl(n, a) ≠ [], ∀ m. m < n0 → lf(m + 1) = nextl(m, lf(m).last) ∧ lf(m) ≠ [] ⊦