⊦ r ∈ st ↔ (∃ sk. r -- sk ∈ st)
∃ sk. r -- sk ∈ st, ∀ rk. rk ∈ st → rk ∈ st0 ∧ st[rk] = st0[rk] ⊦ r ∈ st0
[[CONCRETESIMPRULES : ()], [EXTRATERMS : (st, r)]]