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