static byte[] concatByteArrays_reverseSecond(byte[] a, byte[] b) {
  int la = l(a), lb = l(b), n = la+lb;
  byte[] out = new byte[n];
  arraycopy(a, 0, out, 0, la);
  for i to lb:
    out[n-1-i] = b[i];
  ret out;
}