⊦ (∀ a0. (a0 ∈ st[a, d][b, d0] ↔ a0 ∈ st[b, d0]) ∧ st[a, d][b, d0][a0] = st[b, d0][a0]) ↔ a ∈ st ∧ st[a] = d, a = b