r -- sk0 ∈ st ⊦ ∃ sk1. r = r0 ∧ sk1 = sk ∨ r -- sk1 ∈ st
r = r0 ⊦ ∃ sk1. r = r0 ∧ sk1 = sk ∨ r -- sk1 ∈ st
[[CONCRETESIMPRULES : ()]]