⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2)
⊦ a ∈ x.toSet ↔ a ∈ x
Used simplifier rules: toSet-ext: ⊦ a ∈ x.toSet ↔ a ∈ x;% ⊦ ¬ a ∈ []; (s)% ⊦ ¬ a ∈ ∅; (s) Used terms: ∅, [].toSet