| (n0 ≤ # x0 ⊃ firstn(n0, x0);x0) = (n1 ≤ # x0 + # x1 ⊃ firstn(n1, x0 + x1);x0 + x1) ⊦ n1 < # x0 ∧ n0 = n1, # x0 ≤ n0 ∧ # x0 ≤ n1 ∧ x1 = [], # x0 ≤ n0 ∧ n1 = # x0 ∧ x1 ≠ [] |
(n0 ≤ # x0 ⊃ firstn(n0, x0);x0) = (n1 ≤ # x0 + # x1 ⊃ firstn(n1, x0 + x1);x0 + x1) ⊦ # x0 ≤ n0 ∧ n1 = # x0 ∧ x1 ≠ [], # x0 ≤ n0 ∧ # x0 ≤ n1 ∧ x1 = [], n1 < # x0 ∧ n0 = n1