> won't have a chance to work through it for a week or so

These have been much more in the way of FYI glosses. There's no "suggestion" here to be pursued - just trying to get a deeper understanding of code already written :-)

While I can concoct any number of cases where the add_on() method fails correct rounding, all the ones I dug into turned out to be "oops! went the wrong way" in nearest/even tie cases. By any non-anal measure, its accuracy is excellent.