⊦ ⌜nf⌝ = ⌜a0 '⌝ + ⌜nf0⌝ ↔ nf(0) = a0 ∧ (λ n. nf(n + 1)) = nf0
Used simplifier rules: ⊦ ⌜nf⌝ = ⌜nf0⌝ ↔ nf = nf0; (s)