static double[] subDoubleArray(double[] b, int start, int end default l(b)) {
  start = max(start, 0); end = min(end, l(b));
  if (start == 0 && end == l(b)) ret b;
  if (start >= end) ret new double[0];
  double[] x = new double[end-start];
  System.arraycopy(b, start, x, 0, end-start);
  ret x;
}