svoid replaceConceptAndUpdateRefs(Concept a, Concept b) {
  assertTrue("unlisted", isUnlisted(b));
  Concepts cc = a.concepts();
  assertTrue("listed", cc != null);
  L<Concept.Ref> refs = allBackRefObjects(a);
  b.id = a.id;
  deleteConcept(a);
  registerConcept(cc, b);
  for (Concept.Ref ref : refs)
    ref.set(b);
}