a ∈ a0 + x0, a ∈ y, ∀ a, y. a ∈ y ∧ a ∈ x0 → dups(x0 + y) ⊦ dups((a0 + x0) + y)
Used simplifier rules: list freely generated by [], +;;% ⊦ ¬ a ∈ []; (s)