⊦ S ⊆ S0 ↔ (∀ x. S(x) → S0(x))
∀ x. compl(f)(S)(x) → S(x) ⊦ S(x), ∃ S. S ⊆ f(S) ∧ S(x)
Used terms: S, compl(f)(S)