n < # y, a ∈ y ⊦ #oc(a, y[n, a] -1l a) = #oc(a, y), a = y[n], y = []
Used simplifier rules: ⊦ x[m] ≠ a ∧ m < # x → (a ∈ x[m, a] -1l a ↔ a ∈ x); (s)