Used simplifier rules: in-05: ⊦ a ∈ x → #oc(a, x) ≠ 0;% ⊦ 1 < m ↔ ¬(m = 0 ∨ m = 1); (s)% ⊦ m ≤ n ↔ ¬ n < m; (ss)% ⊦ m < 1 ↔ m = 0; (s)% ⊦ m ≠ n → (m ≤ n ↔ m < n); (s)