static java.util.Timer watchFile(final File f, int interval, final Runnable onChange) {
  ret doEvery(interval, new Runnable {
    FileStatus status = fileStatus(f);
    
    public void run {
      FileStatus s = fileStatus(f);
      if (neq(s, status)) {
        status = s;
        pcallF(onChange);
      }
    }
  });
}