∀ a, b. (a ∈ ins≤(b, x0) ↔ a ∈ x0) ∨ a = b ⊦ a ∈ ins≤(b, a0 + x0) ↔ a ∈ a0 + x0, a = b
Used simplifier rules: ins-e: ⊦ ins≤(a, []) = a ';%list freely generated by [], +;;% ⊦ a ∈ b ' ↔ a = b; (s)% ⊦ ¬ a ∈ []; (s)