⊦ ⌜a '⌝ + ⌜nf⌝ = ⌜λ n. (n = 0 ⊃ a;nf(n -1))⌝
⊦ (nf to m) + nf(m) ' = nf to m + 1, ⌜λ n. (n = 0 ⊃ a;nf(n -1))⌝ = []
Used terms: nf, a