⊦ rk ∈ st ∪ st0 ↔ rk ∈ st ∨ rk ∈ st0
⊦ (∅ ∪ st)[rk] = st[rk]
[[CONCRETESIMPRULES : ([CSIMPSEQ : ⊦ ¬ rk ∈ ∅])]]