n < # x, inv(ixs, n) ⊦ x = [], inv(pushx(n, ixs, x), n + 1)
[Concretesimprules((Csimpnamedseq(list-data,,l, ⊦ # x = 0 ↔ x = []) Csimpnamedforward(nat-basic1,,f, ⊦ m < n → n ≠ 0)))]