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