a ∈ st ⊦ (a0 ∈ st → a0 ∈ st[a, st[a]]) ∧ st[a0] = st[a, st[a]][a0]
Used simplifier rules: In-insert: ⊦ a ∈ st[b, d] ↔ a = b ∨ a ∈ st;