∀ a. st0[a] = (∅)[a] ∨ a ∈ st0 ⊦ a = a0, st0[a] = (∅)[a], a ∈ st0
Used simplifier rules: In-insert: ⊦ a ∈ st[b, d] ↔ a = b ∨ a ∈ st;%At-other: ⊦ a ≠ b → st[b, d][a] = st[a];