# s0 = 1, ∀ a. a ∈ s → a ∈ s0 ⊦ # s = 0, s = s0, s = ∅
Used simplifier rules: Subset: ⊦ s1 ⊆ s2 ↔ (∀ a. a ∈ s1 → a ∈ s2);%size-zero-01: ⊦ s ≠ ∅ → # s ≠ 0;%size-zero: ⊦ # s ≠ 0 → s ≠ ∅;