m < # x, n < # x ⊦ #oc(a, x[m, x[n]][n, x[m]]) = #oc(a, x), x = []
Used simplifier rules: ⊦ # x = 0 ↔ x = []; (s)%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic