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