# s0 + # s1 + 1 < #(s0 ∩ (s1 ++ a)), ∀ s0. ¬ # s0 + # s1 < #(s0 ∩ s1) ⊦ a ∈ s1
Used simplifier rules: ⊦ ¬ a ∈ s → #(s ++ a) = # s + 1; (s)