a ∈ s ⊦ a ∈ s ↔ (∃ s0. (∀ a0. a0 ∈ s ↔ a0 = a ∨ a0 ∈ s0) ∧ ¬ a ∈ s0)
⊦ a ∈ s ↔ (∃ s0. (∀ a0. a0 ∈ s ↔ a0 = a ∨ a0 ∈ s0) ∧ ¬ a ∈ s0), a ∈ s
Used formulas: a ∈ s