Ok, here's what I'm fooling with. I've got a downloaded object that's been constructed with booleans. It's not a model, so it already has quite a few positive/negative groups in it. I want to chop the object in half. If I make the whole object positive, and group it with a negative cube, nothing happens. I unchecked "transfer material of negative boolean". I've tried to find the answer in other threads. I saw someone mention that "solid when boolean rendering" needs to be checked, but evidently that only applies to imported objects. Can anyone help me here? There's gotta be something simple that I'm missing...

OK, nevermind. I figured it out. There was one object embedded deep in there that was still neutral.