a ∈ y, ∀ a, y. a ∈ y → (a ∈ filter(x0, y) ↔ a ∈ x0) ⊦ a ∈ filter(a0 ' + x0, y) ↔ a ∈ a0 ' + x0
Used simplifier rules: ⊦ a + x = a ' + x; (s)