# s = 1, a ∈ s ⊦ ∃ a. ∀ a0. a0 ∈ s ↔ a0 = a
Used simplifier rules: size-one-02: ⊦ # s = 1 ∧ a ∈ s ∧ b ∈ s → a = b;%ne-01: ⊦ a ∈ s → s ≠ ∅;%ne-02: ⊦ a ∈ s → (s = ∅ ↔ false);