a ∈ s0, a0 ∈ s0 ⊦ #(s ∪ s0) = (#(s ∪ s0) - 1) + 1, a ∈ s, 1 ≤ #(s ∪ s0)
Used simplifier rules: n ≤ m ⊦ n0 = m - n ↔ m = n0 + n; (elim) Used terms: #(s ∪ s0), 1, n