void Code::AppendCheckSum()
// postcondition:  if IsValid() then myLength is updated and 
//                 myChars contains the original characters followed
//                 by a dash and the appropriate checksum; otherwise
//                 myLength and myChars are unchanged
{
    if (IsValid())
    {
	int k;
	int sum = 0;
	int len = Length();     // could use myLength here too

	for(k=0; k < len; k++)
	{
	    if (isdigit(myChars[k]))
	    {
		sum += digitToInt(myChars[k]);
	    }
	}
	myChars[myLength]   = '-';
	myChars[myLength+1] = intToDigit(sum % 10);
	myLength += 2;
    }
}