static O[] concatArrays(O[]... arrays) {
  int l = 0;
  for (O[] a : arrays) l += l(a);
  O[] x = new O[l];
  int i = 0;
  for (O[] a : arrays) if (a != null) {
    System.arraycopy(a, 0, x, i, l(a));
    i += l(a);
  }
  ret x;
}