a ∈ s2, ∀ a. a ∈ s1 ∨ a ∈ s2 → a ∈ s ⊦ (a ∈ s1 → a ∈ s) ∧ (a0 ∈ s2 → a0 ∈ s)
∀ a. a ∈ s1 ∨ a ∈ s2 → a ∈ s ⊦ a ∈ s2, (a ∈ s1 → a ∈ s) ∧ (a0 ∈ s2 → a0 ∈ s)
Used formulas: a ∈ s2