a ∈ s, a ∈ x, ∀ a0. ¬ a0 ∈ s ∨ ¬ a0 ∈ x ⊦ s = ∅, # s = 0
Used simplifier rules: in: ⊦ a ∈ list2set(x) ↔ a ∈ x;