⊦ ⌜nf⌝ = ⌜a0 '⌝ + σ ↔ nf(0) = a0 ∧ ⌜λ n. nf(n + 1)⌝ = σ
Used simplifier rules: cons-append: ⊦ a + σ = ⌜a '⌝ + σ;