static Map<Class, Long> cumulativeObjectSizeByClass(Iterable l) {
  new Map<Class, Long> map;
  fOr (O o : l) {
    continue if o == null;
    Class c = o.getClass();
    map.put(c, toLong(map.get(c)) + unsafe_sizeOf2(o));
  }
  ret map;
}