a ∈ st, a0 ∈ st, ∀ a1. a1 ∈ st -- a → a1 ∈ st0 -- a ∧ st[a1] = st0[a1] ⊦ a0 = a, a0 ∈ st0 ∧ st[a0] = st0[a0]