// A is the "tag" type
static <A> MultiMap<Double, A> ufr_brightnessTreeMultiMap(Iterable<Pair<A, IIntegralImage>> taggedImages) {
  MultiMap<Double, A> multiMap = treeMultiMap();
  for (A tag, IIntegralImage img : unpair taggedImages)
    multiMap.put(uRoot(img).averageBrightness(), tag);
  ret multiMap;
}