∀ a. a ∈ s → a ∈ s0 ⊦ a ∈ s ∪ (s0 \ s) ↔ a ∈ s0
Used simplifier rules: ⊦ s1 ⊆ s2 ↔ (∀ a. a ∈ s1 → a ∈ s2); (s)% ⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2); (s)