# x = 1 ⊦ x.head = x.last, x = []
Used simplifier rules: not-zero: 0 < n ⊦ (* n) ≤ m → m ≠ 0; (forward) from specification nat%le-one: ⊦ # x ≤ 1 ∧ x ≠ [] → # x = 1; (forward) from specification list-data