# x + # y ≤ 1 ⊦ x = [], x.tail = [] ∧ y = []
Used simplifier rules: lf-01: ⊦ # x = 1 ↔ x = x.head + [];%fr-07: ⊦ x ≠ [] → (x = x.head + y ↔ x.tail = y);