static <A extends Concept> L<A> deleteOldConcepts(Concepts cc default db_mainConcepts(),
  Class<A> conceptClass, double minAgeInSeconds) {
  new L<A> deleted;
  for (A c : cloneList(list(cc, conceptClass)))
    if (c._modified < now()-toMS(minAgeInSeconds)) {
      print("Deleting concept " + c);
      deleted.add(c);
      cdelete(c);
    }
  ret deleted;
}