| a ∈ s0, a0 ∈ s0, ∀ a0. a0 ∈ s ++ a → a0 ∈ s0, (∀ a. a ∈ s → a ∈ s0) → #(s0 \ s) = # s0 - # s ⊦ #(s0 \ s) - 1 = # s0 - # s + 1, a ∈ s |
a ∈ s0, a0 ∈ s0, ∀ a0. a0 ∈ s ++ a → a0 ∈ s0, (a1 ∈ s → a1 ∈ s0) → #(s0 \ s) = # s0 - # s ⊦ #(s0 \ s) - 1 = # s0 - # s + 1, a ∈ s