a ∈ y, ∀ a, y. a ∈ y → (a ∈ filter(x0, y) ↔ a ∈ x0) ⊦ a ∈ filter(a0 + x0, y) ↔ a ∈ a0 + x0
Used simplifier rules: filt-e: ⊦ filter([], y) = [];%list freely generated by [], +;;% ⊦ ¬ a ∈ []; (s)