⊦ r ∈ st ↔ (∃ sk. r -- sk ∈ st)
⊦ r -- sk ∈ st → r ∈ st
r -- sk0 ∈ st, r -- sk1 ∈ st ⊦ r = r0, ∃ sk0. r -- sk0 ∈ st -- (r0 -- sk)
[[CONCRETESIMPRULES : ()]]