| x.head = a, # x ≤ # y0 + 1, x.tail ⊑ y0 + z, ∀ x, z. # x ≤ # y0 ∧ x ⊑ y0 + z → (∃ y1. y0 = x + y1) ⊦ x = [], ∃ y. a ' + y0 = x + y |
# x ≤ # y0 + 1, x.tail ⊑ y0 + z, ∀ x, z. # x ≤ # y0 ∧ x ⊑ y0 + z → (∃ y1. y0 = x + y1) ⊦ x = [], ∃ y. x.head ' + y0 = x + y