⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2)
∀ b. b ∈ s1 ↔ b = a ∨ b ∈ s0 ⊦ ∀ a0. a0 ∈ s1 ↔ a0 ∈ s0 ++ a
Used terms: s0 ++ a, s1