Used simplifier rules: subdef: ⊦ s1 ⊆ s2 ↔ (∀ a. a ∈ s1 → a ∈ s2);%sub-del-04: ⊦ s ⊆ s0 -- a ↔ s ⊆ s0 ∧ ¬ a ∈ s;