x = a0 + y, n < # x ⊦ x[n, a] = firstn(n, x) + a ' + restn(n, y), x = []
Used simplifier rules: x ≠ [] ⊦ a = x.first ∧ y = x.rest ↔ x = a + y; (elim) Used terms: x, a0, y