# s < # s0 ⊦ ∃ a. a ∈ s0 ∧ ¬ a ∈ s
Used simplifier rules: sub-size: ⊦ s ⊆ s0 → ¬ # s0 < # s;%neq-01: ⊦ m ≠ n → (¬ m < n ↔ n < m); from specification nat-basic1 Used terms: s, s0