code: Object getValue(
        MatchSpaceKey msg,
        Object contextValue)
        throws MatchingException, BadMessageFormatMatchingException
      {
        return msg.getIdentifierValue(id, false, contextValue, false);
      }
docs: Default implementation of getValue(), SetVal subclasses will
  override
  
  @param msg
  @param contextValue
  @throws MatchingException
  @throws BadMessageFormatMatchingException