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