# y = # y + # z - n, n ≤ # y + # z ⊦ ∃ y0, z0. y + z = y0 + z0 ∧ # z0 = n
Used simplifier rules: len: ⊦ #(x + y) = # x + # y;%a: ⊦ (x + y) + z = x + y + z;