Used simplifier rules: Keys: ⊦ a ∈ st.keys ↔ a ∈ st;%ne-01: ⊦ a ∈ s → s ≠ ∅; (forward) from specification set-basic%sub-in: ⊦ a ∈ s ∧ s ⊆ s0 → a ∈ s0; (forward) from specification set-basic