⊦ ⌜x⌝ + ⌜nf⌝ = ⌜λ n. (n < # x ⊃ x[n];nf(n - # x))⌝
⌜nf⌝ = ⌜λ n. (n < # x ⊃ x[n];nf0(n - # x))⌝ ⊦ x = nf to # x ∧ nf0 = nf from # x
Used terms: nf0, x