Used simplifier rules: notin: ⊦ ¬ a ∈ x → #oc(a, x) = 0;%in-05: ⊦ a ∈ x → #oc(a, x) ≠ 0;%lez-01: ⊦ m ≤ 0 ↔ m = 0; from specification nat Used terms: a