static short[] concatShortArrays(L<short[]> arrays) {
  int l = 0;
  for (short[] a : unnull(arrays)) l += l(a);
  short[] x = new short[l];
  int i = 0;
  for (short[] a : unnull(arrays)) if (a != null) {
    System.arraycopy(a, 0, x, i, l(a));
    i += l(a);
  }
  ret x;
}