⊦ st \ (s ++ a) = (st \ s) -- a
∀ a, st. ¬ a ∈ s0 → (st \ s0)[a] = st[a] ⊦ ¬ a ∈ s0 ++ a0 → ((st \ s0) -- a0)[a] = st[a]
Used terms: a0, s0, st