static <A extends Concept> A replaceConceptAndUpdateRefs(Concept a, A b) {
  assertTrue("object to replace with must be unlisted", isUnlisted(b));
  Concepts cc = a.concepts();
  assertTrue("object to replace must be listed", cc != null);
  L<Concept.Ref> refs = allBackRefObjects(a);
  b.id = a.id;
  deleteConcept(a);
  cc.register_phase2(b);
  cset(a, meta_migratedTo := b); // for objects that may have had direct pointers
  for (Concept.Ref ref : refs)
    ref.set(b);
  ret b;
}