⊦ S ⊆ S0 ↔ (∀ x. S(x) → S0(x))
∀ x. compl(f)(S)(x) → S(x) ⊦ S(x), ∀ x. ¬ S(x) → f(λ x0. ¬ S(x0))(x)
Used terms: f(λ x0. ¬ S(x0)), λ x0. ¬ S(x0)