⊦ st1 = st2 ↔ (∀ a. (a ∈ st1 ↔ a ∈ st2) ∧ st1[a] = st2[a])
∀ a0. (a0 ∈ st[a, (∅)[a]] ↔ a0 ∈ st) ∧ st[a, (∅)[a]][a0] = st[a0] ⊦ a ∈ st
Used terms: st, st[a, (∅)[a]]