⊦ rk ∈ st ∪ st0 ↔ rk ∈ st ∨ rk ∈ st0
st .refs ∩ st0 .refs = ∅, rk ∈ st ⊦ st[rk] = (st ∪ st0)[rk]
[[CONCRETESIMPRULES : ()]]