st[a, d] = st0[a, d0] ⊦ a ∈ st0, ∃ st0. st0[a, d] = st[a, d] ∧ ¬ a ∈ st0
Used simplifier rules: In-insert: ⊦ a ∈ st[b, d] ↔ a = b ∨ a ∈ st;