∀ a0. a0 ∈ s ++ a → a0 ∈ s0, (a0 ∈ s → a0 ∈ s0) → #(s0 \ s) = # s0 - # s ⊦ #(s0 \ s) = # s0 - #(s ++ a), a ∈ s0 \ s