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

static double[] concatDoubleArrays(double[]... arrays) {
  ret concatDoubleArrays(asList(arrays));
}