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