static <A extends Concept> L<A> mergeConceptLists(Collection<A> l1, Collection<A> l2) {
  new HashSet<Long> ids;
  new L<A> out;
  for unnull (A a : l1) if (ids.add(a.id)) out.add(a);
  for unnull (A a : l2) if (ids.add(a.id)) out.add(a);
  ret out;
}