// slightly experimental - also uses allChanged() which might not be
// so good with future logging
svoid swapConceptIDs(Concept a, Concept b) {
  Concepts concepts = a._concepts;
  if (concepts != b._concepts) fail("Concepts belong to different set");
  long id1 = a.id, id2 = b.id;
  if (concepts == null) { a.id = id2; b.id = id1; ret; }
  lock concepts.lock;
  a.id = id2;
  b.id = id1;
  concepts.concepts.put(id2, a);
  concepts.concepts.put(id1, b);
  a.change();
  b.change();
}