static L<ConceptShadow> filterConceptShadows(L<ConceptShadow> l1, L<ConceptShadow> l2) {
  new L<ConceptShadow> out;
  int i1 = 0, i2 = 0;
  while (i1 < l(l1) && i2 < l(l2)) {
    ConceptShadow s1 = l1.get(i1), s2 = l2.get(i2);
    long id1 = s1.id(), id2 = s2.id();
    if (id1 == id2)
      out.add(s1);
    else if (id1 < id2)
      ++i1;
    else
      ++i2;
  }
  ret out;
}