// raise flag to stop
static Flag tailFile(final File file, final int interval, fO onData) {
  final new Flag flag;
  thread "Tail File" {
    long l = 0;
    repeat with ms sleep interval {
      pcall {
        long l2 = l(file);
        if (l2 < l) l = 0;
        if (l2 != l) pcall {
          S text = null;
          pcall {
            text = loadTextFilePart(file, l, l2);
          }
          if (nempty(text))
            callF(onData, text);
          l = l2;
        }
      }
      if (flag.isUp()) break; // break AFTER we checked for new data
    }
  }
  ret flag;
}