n < #oc(a, x), a ∈ x ⊦ #oc(a, x) = 1, dups(x)
Used simplifier rules: oc-zero: ⊦ #oc(a, x) = 0 ↔ ¬ a ∈ x;%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic1