static byte[] hexToBytes(S s) {
  if (odd(l(s))) fail("Hex string has odd length: " + quote(shorten(10, s)));
  int n = l(s) / 2;
  byte[] bytes = new byte[n];
  for (int i = 0; i < n; i++) {
    int a = parseHexChar(s.charAt(i*2));
    int b = parseHexChar(s.charAt(i*2+1));
    if (a < 0 || b < 0)
      fail("Bad hex byte: " + quote(substring(s, i*2, i*2+2)) + " at " + i*2 + "/" + l(s));
    bytes[i] = (byte) ((a << 4) | b);
  }
  ret bytes;
}