⊦ #(s ∪ s0) ≤ # s ↔ (∀ a. a ∈ s0 → a ∈ s)
Used simplifier rules: Subset: ⊦ s1 ⊆ s2 ↔ (∀ a. a ∈ s1 → a ∈ s2); from specification set-basic