a ∈ y, ∀ a0. #oc(a0, y) ≤ #oc(a0, y -1l a), ∀ a0. #oc(a0, y -1l a) ≤ #oc(a0, y) ⊦
Used simplifier rules: ⊦ ¬ a ∈ x → x -1l a = x; (s)% ⊦ m ≤ m; (s)