code: protected static char calcChecksumChar (@Nonnull final String sMsg, @Nonnegative final int nLength)
    {
      ValueEnforcer.notNull (sMsg, "Msg");
      ValueEnforcer.isBetweenInclusive (nLength, "Length", 0, sMsg.length ());
  
      return asChar (calcChecksum (sMsg.toCharArray (), nLength));
    }
docs: Calculates the check character for a given message
  
  @param sMsg
  the message
  @param nLength
  The number of characters to be checked. Must be &ge; 0 and &lt;
  message.length
  @return char the check character