⊦ ⌜x + y⌝ = ⌜x⌝ + ⌜y⌝
⊦ ⌜x + x0⌝ + ⌜nf⌝ = ⌜λ n. (n < # x ⊃ x[n];(n < # x0 + # x ⊃ x0[n - # x];nf(n - # x - # x0)))⌝
Used terms: x0, x