a ∈ firstn(n, a0 ' + x0), n ≤ pos(a, a0 ' + x0), ∀ a1, n0. ¬ a1 ∈ firstn(n0, x0) ∨ ¬ n0 ≤ pos(a1, x0) ⊦
Used simplifier rules: ⊦ a + x = a ' + x; (s)