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