⊦ a ∈ st ↔ (∃ d, st0. st = st0[a, d] ∧ ¬ a ∈ st0)
a ∈ st[a, d] ↔ (∃ d0, st0. st[a, d] = st0[a, d0] ∧ ¬ a ∈ st0) ⊦ ∃ st0. st0[a, d] = st[a, d] ∧ ¬ a ∈ st0
Used terms: a, st[a, d]