⊦ s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2)
st.min ∈ st, ∀ a. a ∈ st.keys ↔ a ∈ ∅ ⊦ st = ∅, ¬ a ∈ st ∧ st[a] = (∅)[a]
Used terms: ∅, st.keys