∀ st, a. (st \ s0)[a] = st[a] ∨ a ∈ s0 ⊦ (st \ (s0 ++ a0))[a] = st[a], a ∈ s0 ++ a0
Used simplifier rules: Diff-base: ⊦ st \ ∅ = st;%set generated by ∅, ++;;