a ∈ a0 + x0, ∀ a. a ∈ x0 → #oc(a, x0) = 1 ∨ dups(x0) ⊦ #oc(a, a0 + x0) = 1, dups(a0 + x0)
Used simplifier rules: oc-e: ⊦ #oc(a, []) = 0;%list freely generated by [], +;;% ⊦ ¬ a ∈ []; (s)