⊦ (a0 ∈ st[a, d] -- a ↔ a0 ∈ st -- a) ∧ (st[a, d] -- a)[a0] = (st -- a)[a0]
Used simplifier rules: Extension: ⊦ st1 = st2 ↔ (∀ a. (a ∈ st1 ↔ a ∈ st2) ∧ st1[a] = st2[a]);