a ≠ a → (a ∈ s ↔ a ∈ s0), ∀ a0. a0 ≠ a → (a0 ∈ s ↔ a0 ∈ s0) ⊦ a0 ∈ s ↔ a0 ∈ s0, a ∈ s0, a ∈ s
Used terms: a