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