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