# y = n - 1, n ≤ 1 + (n - 1) + # z ⊦ ∃ y0, z0. a ' + y + z = y0 + z0 ∧ # y0 = n
Used simplifier rules: len: ⊦ #(x + y) = # x + # y;%a: ⊦ (x + y) + z = x + y + z;