Used simplifier rules: subdef: ⊦ s1 ⊆ s2 ↔ (∀ a. a ∈ s1 → a ∈ s2);%del-rax: ⊦ a ∈ s -- b ↔ ¬ ¬(a ≠ b ∧ a ∈ s);%Delete: ⊦ a ∈ s -- b ↔ a ≠ b ∧ a ∈ s;