⊦ nf(n0 + n) = (n0 = 0 ⊃ nf(n);nf((n + 1) + (n0 - 1)))
Used simplifier rules: get-from-inf: ⊦ ((nf from n))(m) = nf(m + n);% ⊦ ⌜nf⌝ = ⌜nf0⌝ ↔ nf = nf0; (s)% ⊦ n -1 = n - 1; (s)