⊦ s1 ⊆ s2 ↔ (∀ a. a ∈ s1 → a ∈ s2)
s1 ∩ s2 = ∅, ∀ a. a ∈ s0 → a ∈ s1 ⊦ #(s0 ∩ s2) = 0, s0 ∩ s2 = ∅
Used terms: s1, s0