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