∀ a, b. (a ∈ ins≤(b, x0) ↔ a ∈ x0) ∨ a = b ⊦ a ∈ ins≤(b, a0 ' + x0) ↔ a ∈ a0 ' + x0, a = b
Used simplifier rules: ⊦ a + x = a ' + x; (s)