⊦ ns = ns0 ↔ (∀ n. n ∈ ns ↔ n ∈ ns0)
n ≤ m ⊦ ∀ n0. n0 ∈ below(ns -- m, n) ↔ n0 ∈ below(ns, n)
Used terms: below(ns, n), below(ns -- m, n)