code: function fromHex(hex, binary, offset) {
    var length = hex.length / 2;
    if (offset === undefined) {
      offset = 0;
      if (binary === undefined) binary = create(length);
    }
    var j = 0;
    for (var i = 0; i < length; i++) {
      binary[offset + i] = (codeToNibble(hex.charCodeAt(j++)) << 4)
                         |  codeToNibble(hex.charCodeAt(j++));
    }
    return binary;
  }
docs: Like copy, but decode from a hex string