x -l a ⊆m y -l a ⊦ #oc(a, x -1l a) ≤ #oc(a, y) ↔ #oc(a, x) ≤ #oc(a, y) + 1
Used simplifier rules: ⊦ (x -1l a) -l a = x -l a; (s)