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