# nσ = ⌜m⌝ + ⌜1⌝ , n < nσ[m], stutf(nσ) ⊦ nσ[m] = 0, ⌜n⌝ < leninv(nσ)
Used simplifier rules: f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic1