sclass G22ForegroundFinder {
  // input
  settable L<IImageRegion> allRegions;
  
  // internal
  gettable Rect borderRect;
  
  // output
  Set<IImageRegion> foregroundRegions;
  
  Set<IImageRegion> foregroundRegions() {
    run();
    ret foregroundRegions;
  }
  
  run {
    if (foregroundRegions != null) ret;
    borderRect(mergeRects(map(allRegions, r -> r.getBounds())));
    for (allRegions)
    foregroundRegions = new LinkedHashSet;
    
  }
  
  swappable bool isForegroundRegion(IImageRegion region) { throw unimplemented(); }
}