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

Began life as a copy of #1032902