⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2)
∀ a0. a0 ∈ s0 ∩ s ↔ a0 ∈ ∅ ++ a, a ∈ s ⊦ # s = 0, s = ∅, a ∈ s0
Used terms: ∅ ++ a, s0 ∩ s