⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2)
∀ a. a ∈ list2set(x) ∩ list2set(y) ↔ a ∈ ∅, a ∈ y, a ∈ x ⊦
Used terms: ∅, list2set(x) ∩ list2set(y)