⊦ r ∈ st ↔ (∃ sk. r -- sk ∈ st)
⊦ (∃ sk0. r -- sk0 ∈ st[r0 -- sk, val]) ↔ r = r0 ∨ (∃ sk. r -- sk ∈ st)
[[CONCRETESIMPRULES : ()], [EXTRATERMS : (st, r)]]