# s = 1 ⊦ ∃ a. ∀ a0. a0 ∈ s ↔ a0 = a
Used simplifier rules: Extension: ⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2);%in-04: ⊦ a ∈ ∅ ++ b ↔ a = b;