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