⊦ st1 = st2 ↔ (∀ a. (a ∈ st1 ↔ a ∈ st2) ∧ st1[a] = st2[a])
∀ a. (a ∈ st0 ∪ st1 ↔ a ∈ ∅) ∧ (st0 ∪ st1)[a] = (∅)[a] ⊦ st0 = ∅ ∧ st1 = ∅
Used terms: ∅, st0 ∪ st1