⊦ ¬ is_valid_ref(r, st) → ¬ has_reffield(r, st)
¬ ¬ is_valid_ref(r0, st), has_reffield(r0, st) ⊦ has_reffield(r0, st -- r), r0 = r
[[CONCRETESIMPRULES : ()], [EXTRATERMS : (st, r0)]]