static LPair<Pt> assignEachPointToClosestNeighbor(Cl<Pt> l1, Cl<Pt> l2) {
  new LPair<Pt> pairs;
  for (Pt a : l1) {
    if (empty(l2)) break;
    Pt b = lowestAccordingTo(l2, x -> ptDistanceSquared(a, x));
    l2.remove(b);
  }
  ret pairs;
}