⊦ s1 ⊆ s2 ↔ (∀ a. a ∈ s1 → a ∈ s2)
∀ a0. a0 ∈ s → a0 ∈ s0 ⊦ a ∈ s0, a ∈ s, ∀ a. a ∈ s → a ∈ s0, s0 ++ a ⊆ s0
Used terms: s0, s