a ∈ s0, s ⊆ s0, # s0 ≤ # s ⊦ a ∈ s
Used simplifier rules: sub-in: ⊦ a ∈ s ∧ s ⊆ s0 → a ∈ s0;%ne-01: ⊦ a ∈ s → s ≠ ∅;