svoid test_regionsRoughlyByDecreasingSize() {
  BWImage img = bwImageFromString([[
    ___xxx
    ______
    __xx_x
    __xx_x
    ______
    _xx___
  ]]);
  FastRegions_BWImage regions = new(img);
  regions.run();
  printStruct(regions.regionsBySize);
  for (i : regions.regionsRoughlyByDecreasingSize())
    print(i + " - " + regions.regionSize(i));
}