a ∈ s1, a ∈ s1 ∨ a ∈ s2, ∀ a. a ∈ s1 → a ∈ s, ∀ a. a ∈ s2 → a ∈ s ⊦ a ∈ s
a ∈ s1 ∨ a ∈ s2, ∀ a. a ∈ s1 → a ∈ s, ∀ a. a ∈ s2 → a ∈ s ⊦ a ∈ s1, a ∈ s
Used formulas: a ∈ s1