⊦ x ⊑ y ↔ (∃ z. x + z = y)
# x1 + n < # x0, ∃ z. x0 + z = x1 + (n ≤ # x2 ⊃ firstn(n, x2);x2) ⊦ x0 = []
Used terms: x1 + (n ≤ # x2 ⊃ firstn(n, x2);x2), x0