| ⊦ # prefixn(n, x) ≤ n |
⊦ prefixn(n, x) = (n ≤ # x ⊃ firstn(n, x);x)
[Concretesimprules((Csimpnamedseq(prefixn,,, ⊦ prefixn(n, x) = (n ≤ # x ⊃ firstn(n, x);x)) Csimpnamedseq(list-del,,first-length, ⊦ # firstn(n, x) = n) Csimpnamedseq(nat-basic1,,ls-02, ⊦ m < n → ¬ n < m) Csimpnamedforward(nat-basic1,,f, ⊦ m < n → n ≠ 0) Csimpnamedseq(nat,,lels-01, ⊦ ¬ m ≤ n ↔ n < m)))]