s0 ⊆ S, ∀ w. ¬ w ∈ s0 ⊦ # s0 = 0, s0 = ∅
[[CONCRETESIMPRULES : ([CSIMPFORWARD : ⊦ # s ≠ 0 → s ≠ ∅], [CSIMPFORWARD : ⊦ s ≠ ∅ → # s ≠ 0])]]