n ≤ m ⊦ n0 ≠ m ∧ n0 ∈ ns ∧ n0 < n ↔ n0 ∈ below(ns, n)
Used simplifier rules: ⊦ n ∈ ns -- n0 ↔ n ≠ n0 ∧ n ∈ ns; (s)