| (m ≤ # x ⊃ firstn(m, x);x) = y + z ⊦ y = x, (# y < m ⊃ # y;m) ≤ # x |
(m ≤ # x ⊃ firstn(m, x);x) = y + z, # x < (# y < m ⊃ # y;m) ⊦ y = x
Used simplifier rules: ⊦ m < n → n ≠ 0; (s)%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic% ⊦ m ≤ n ↔ ¬ n < m; (ss)