list2set(x) ∩ s = ∅, a ∈ s, a ∈ x ⊦ s = ∅, # s = 0
Used simplifier rules: ⊦ s ≠ ∅ → # s ≠ 0; (forward)% ⊦ a ∈ s → s ≠ ∅; (forward)% ⊦ s ∩ s0 = s0 ∩ s;% ⊦ (s ∩ s0) ∩ s1 = s ∩ s0 ∩ s1;